Springtime in the Northern Hemisphere brings clay-court tennis -- that delightful pastime played outdoors on court surfaces gentle to the human skeleton. Coming to an end is the long run of hard-court tennis, now concluding at Miami.
 
The leading pros tend to separate into two groups -- (1) players whose skills are best suited to hard courts and (2) those most successful on clay. Here are the hard-court and clay-court leaders for 2010, measured by ATP ranking points. Note that only two players are common to both lists.
 
Hard-court leaders, 2010
1. Federer
2. Nadal
3. Murray
4. Djokovic
5. Soderling
6. Roddick
 
Clay-court leaders, 2010
1. Nadal
2. Ferrer
3. Verdasco
4. Soderling
5. Almagro
6. Ferrero
 
Various factors explain why hard-court and clay-court rankings are often so different.
 
Foot traction is generally good on hard courts. But because clay courts are slippery, clay-court tennis increases the premium on athletic movement, where the player's body requires strong control in starting, stopping, and changing directions. Very tall players are disadvantaged, while players who grew up playing on clay seem to move naturally and are comfortable in sliding into shots.
 
Another difference is in the importance of strong serving ability. Good serving is valuable on both hard and clay courts. But hard courts enable serves to be more forceful and make aces more frequent, thereby helping very strong servers, more so than on clay. Also the weak server is often especially disadvantaged on clay courts because softish second serves become more vulnerable to attack.
 
Players who succeed on clay courts are often heavy users of topspin, where the ball's rotational momentum does several things. Extreme ball rotation, i.e., heavy overspin, (1) reacts with the air to make hard-hit shots curve downward and land inside the boundaries, (2) adds to the overall momentum that the opponent must overcome in a typical reply, and (3) reduces the horizontal slowing-down in the bounce, thus assisting the ball's forward and upward velocities in bouncing and making opponent's reply more difficult. The first two effects happen on all surfaces, but the third is by far the greatest on clay.
 
A decade ago and before, champions on hard courts were typically those with the strongest net game. The U.S. Open in 1978 became the first Slam to be played on hard courts, and its future champions were often superior net artists -- indeed serve-and-volley players, who came to net regularly behind serves. Serve-and-volleyers McEnroe, Becker, Edberg, Sampras, and Rafter captured the trophy in New York a total of 13 times in the first 21 years of hard-court play. Since then, however, as racket technology has advanced, serve-and-volley play has become only an occasional variant, used only slightly more frequently on hard courts than on clay. The hard-court and the clay games have become more alike in that both are played largely from back court.
 
But it is still possible to detect the two populations, as suggested in the hard-court and clay-court lists shown above and also in the lists of recent champions at the major tournaments. Players from South America and continental Europe, especially Spain, dominate on clay but are seldom winners on hard courts. Of the two megastars of our time, Federer in 2010 was still the best on hard courts, Nadal on clay.
 
THE PUREST OF THE PURE
 
We begin by seeking what are sometimes called the clay-court specialists. We identify them by comparing each player's career W-L performance on clay with his past performance on hard courts (including carpet). Our sample consists of the top 80 in the current 12-month men's rankings, where we exclude about a dozen individuals whose career records are too brief in one of the realms for plausible comparisons.
 
Emerging from our computer are the purest of the pure clay courters -- six confirmed and extreme clay-court specialists. The more a player's winning percentage on clay exceeds his hard-court percentage, the higher his "h-c index." The high-index players are not necessarily the best clay courters, but they are the ones whose skills are most slanted toward clay. As a group they suggest a composite picture of today's archetype clay artist.
 
#1. Filippo Volandri, age 29, height 6-0, c-h index +41.45. This veteran Italian shows 58.1% career winning percentage on clay against 16.7% on hard courts. His playing activity, not surprisingly, has been overwhelmingly on clay (244 main-tour and slam matches on clay, only 66 on hard courts), a tendency increasingly seen in the last year or so. His several tournament crowns and final-round appearances have all been on clay, and by far his greatest success at Slams came when he reached the fourth round at Garros 2007. Despite his lack of success on nonclay surfaces, Filippo attained the world's year-end top fifty in every year 2003-2007, and he is currently ranked #80 in the ATP rankings. Last year his best showing came on clay at Italian Open, where he won two matches before losing closely to Gulbis.
 
#2. Thomaz Bellucci, 23, 6-2, index +22.45. Well behind Volandri in his c-h index but slightly ahead of Filippo in his 61.4% winning percentage on clay is this Brazilian lefty. Thomaz won three matches at last year's Garros, reaching the round of 16, losing then to Nadal. He scored three wins at Acapulco recently, defeating Verdasco and losing to Almagro. His scheduling has been more balanced than Volandri's, as his appearances on hard courts have been only slightly fewer than on clay.
 
#3. Potito Starace, 29, 6-2, index +21.55. This countryman and contemporary of Volandri shows similar, though less extreme, bias toward clay in his past activity. His clay winning percentage is also similar to Volandri's but his c-h index is lower because of Potito's greater success on hard courts. Having grown up playing on clay, he finds sliding a natural way of moving. He believes he obtains explosive power from his legs when sliding and cannot do so on hard courts. He briefly reached the world's third ten in 2007. He won five of his nine matches in the 2011 Latin American clay circuit.
 
#4. Juan Monaco, 26, 6-1, index +21.01. Always a determined and hard-working opponent, Monaco has remained in or just outside the world's second ten for several years. Born and grown in Argentina, he has played almost twice as often on clay courts as on hard, and all ten of his tournament crowns or runner-up finishes have come on clay. He beat Baghdatis at Garros 09 before a close loss to Tsonga, having beaten Murray and Cilic on clay at Rome.
 
#5. Nicolas Almagro, 25, 6-0, index +20.56. Almagro's excellent results in the 2011 Latin American clay sequence gave this Spanish stalwart an early jump in the new year's clay rankings. Meanwhile his improving success in reaching late rounds on hard courts has also furthered his overall ranking, which has been consistently in the second ten of the ATP standings. His playing style features spectacular and relentless heavy hitting, including fierce power in serving and stroking.
 
#6. Albert Montanes, 30, 5-9, index +20.41. Over the years Albert has played nearly three times as many matches on clay as on hard courts. All five of his tournament triumphs have come on clay, including Estoril and Stuttgart in 2010. (At Estoril he beat Federer.) Garros has generally been his best Slam, but he reached the fourth round at U.S. Open 2010, losing to Soderling in four sets. He surprised Almagro on a hard court at Indian Wells 2011.
 
Five of our six extreme clay specialists, above, are between 6-0 and 6-2 in height. All are from clay-court nations -- Spain, Italy, Argentina, and Brazil. Generally they compete far more often on clay than on other surfaces. Several are well along in their careers, though at age 30 Montanes seems to be on an upward run.
 
As our list descends further in values of c-h index, the next dozen or so members show similar patterns though in lesser extremes. High c-h players include prominent members of the Armada -- Nadal at c-h index +17.46, Ferrero at +16.74, Gimeno Traver at +15.28, Robredo at +14.01, and Ferrer at +10.49. Among others are Hanescu of Hungary at +18.45, Serra and Chardy of France at +17.46 and +12.51, respectively, and Chela of Argentina at +10.14. A surprising member is Andrey Golubev, who grew up near the Volga River but changed his tennis nationality to represent Kazakhstan, which borders Russia slightly east of Andrey's birthplace. Unlike the others highlighted here, Andrey has competed mainly on hard courts, which he says he favors. But he won German Open on clay last year in Hamburg, so that his six wins there without loss drastically raised to +18.83 his otherwise thinly based clay-court percentage.
 
All players having strongly positive c-h values -- our six extremes along with the better-known others also noted -- will be dangerous opponents for higher-seeded players in the major clay events ahead. Seedings at pro events are based on both nonclay and clay results, so that those higher-seeded stars who must face our clay specialists on their favored surface will have been unlucky victims of the draw.
 
HARD-COURTERS AND GENERALISTS
 
Conversely, those players having strongly negative c-h values are the extreme hard-courters, who in the coming clay events should be less dangerous opponents than their official ranking and seeding would indicate. Our six extreme hard-courters are headed by Sergiy Stakhovsky.
 
#1. Sergiy Stakhovsky, age 25, height 6-4, c-h index -23.50. Tall, Ukrainian-born Stakhovsky shows 55.3% winning percentage on hard courts, 31.8% on clay, where his hard-court appearances outnumber those on clay by nearly three times. The pattern was most extreme last year, when he scored only three match wins in playing eight clay tournaments. But on hard courts he scored two main-draw victories at both U.S. Open 2010 and Australian Open 2011. His best results have been on grass.
 
#2. Andy Murray, 23, 6-3, index -22.20
#3. Michael Berrer, 30, 6-4, index -16.47
#4. Mardy Fish, 29, 6-2, index -15.55
#5. John Isner, 25, 6-9, index -15.08
#6. Ben Becker, 29, 5-10, index -13.82
 
The average height of our six extreme hard-courters is, at 6-3, three inches taller than the average of our six extreme clay artists. The difference would be more marked if other hard-courters beyond the extreme six were included -- the likes of Querrey, Petzschner, Troicki, and Llodra. Not included in our sample population, as his ranking is outside the top 80, was Ivo Karlovic, at height 6-10 and an h-c index of -7.67.
 
Most former Slam champions who are still active show hard-court c-h values, as listed below. Roddick is strongly in the hard-court direction, but the others fall close to a range in h-c where the term "generalist" might begin to apply. We calculate Andy Roddick at index -9.53, Lleyton Hewitt at -5.08, Novak Djokovic at -4.94, and Roger Federer at -4.81
 
Our formula also tells us which players have been the truest generalists -- i.e., those whose c-h index is closest to zero:
 
Ernests Gulbis, index +0.41
Juan Martin del Potro, -0.49
David Nalbandian, +0.67
Philippe Kohlschreiber, -0.77
Richard Gasquet, +0.80
 
CLAY RACE 2011
 
The four-tournament clay-court circuit in Latin America ended at Acapulco in late February. March then saw the first round of the year's Davis Cup play, where winning players receive individual ranking points. Several of these meetings took place on clay. Here, then, are the current (March 24), standings in points earned on clay in 2011, both in the Latin American sequence and in Davis Cup.
 
1. Nicolas Almagro, 800 ranking points
2. David Ferrer, 500
3. Tommy Robredo, 360
4. Alex Dolgopolov, 330
5. Juan Ignacio Chela, 240
5. Santiago Giraldo, 240
 
The prime clay-court season starts after the Sony-Ericsson in Miami, now in progress. The U.S. Clay Courts championships in Houston (4-10 April) usually showcases the leading U.S. players. Andy Roddick has won the event three times and Mardy Fish won in 2006, but since that year the champions have been from other lands. The green clay formerly used at the event has been replaced by European-style red clay. Meanwhile in the same week, many of the European artists will gather in Casablanca. Wawrinka won there last year, where his semi-final opponent in a split-setter was Starace, one of our extreme six.
 
The action then shifts exclusively to Europe. Monte Carlo, in April's second week, offers 1,000 ranking points to the winner and should bring out most of the world's top players, including Nadal and Federer but apparently not Djokovic. Rafa is the perennial champion there, and he is the overwhelming favorite to win again this year. The tour reconvenes in Barcelona the following week, where Verdasco will be the defending champion. Fernando has a clay-leaning c-h index at +6.10, though his past hard-court appearances have been slightly more frequent than on clay. Nadal is expected at Barcelona along with Soderling, Murray, Ferrer, and Almagro. The outcomes at Monte Carlo and Barcelona should drastically change the appearance of the year's clay-court standings.
 
Several red-clay tournaments will occupy April's last week, and the big 1,000-point tournaments in Madrid and Rome will then follow in sequence. Roland Garros, 22 May-5 June, brings 2,000 points to its champion. Finally, ending the year's clay-court race will be five lesser tournaments in three weeks in Europe in July, directly after Wimbledon.
 
THE CONTENDERS
 
Probably the strongest employer of topspin in tennis history is Rafael Nadal, whose powerful physique helps him generate rotational velocities measured to be far greater than those of any other contemporary player. There are many other strengths in Rafa's arsenal, but his extreme and relentless overspin is the one that most distinguishes him from all others. Probably no other player can defeat a fully healthy Nadal on clay, where that overspin has its greatest effect.
 
Rafa's magnificent career includes his triumphing on clay at Garros five times in the last six years and at Italian Open the same. In this column one year ago our analysis placed him essentially co-equal with Bjorn Borg as the greatest clay-courter of tennis history. Since then Rafa has captured his fifth Garros crown and swept through the other prime clay events of 2010. Another year of clay achievements like last year's would confirm Rafa's primacy over Borg seemingly irrefutably.
 
Our extreme clay-court specialists are likely to claim attention and collect significant ranking points in the early-spring 250-point events, where few of the hard-courters will be seen. The likely champions even there will be from Spain's armada of clay experts, probably including Verdasco and Ferrer. The 1,000-point events will bring forth Nadal and the top generalists and hard-courters, among them Federer, Djokovic, del Potro, Soderling, and Roddick, all capable of strong performances on clay in best-of-three-set competition, perhaps even pushing Nadal to produce his best. There should be many interesting match-ups, which can be examined and dissected in the context of the c-h index.
 
Based on our c-h compilation, of the players likely to receive the sixteen seeded places at the coming 1,000-point clay events, those most likely to surpass their seeded level are Nicolas Almagro, David Ferrer, Stan Wawrinka, and Fernando Verdasco. Those least likely to do so and who are most likely to lose prior to reaching their seeded level are Andy Murray, Mardy Fish, Viktor Troicki, Robin Soderling, and Jo-Wilfried Tsonga.
 
THE WOMEN CLAY-COURTERS
 
The women's clay calendar for 2011 parallels the men's, including two early tournaments in Latin America, eleven tournaments in the prime clay season culminating at Garros, and finally a two-week sequence in July. The women's champion wins 2,000 points at Garros, 1,000 points at the Premier Mandatory tournament in Madrid, 900 points at the Premier Five champion in Rome, and 470 points at each of the Premier tournaments in Stuttgart, Brussels, and Charleston.
 
The likely women's champion on clay remains far from clear. The Williams sisters have been largely absent because of injuries, and Henin and Dementieva have retired. Kim Clijsters won Australian Open early this year but was then was forced to the sidelines at Indian Wells with shoulder trouble. Former #1 rankers Sharapova and Safina have been struggling to regain former eminence. Thus the apparent heiress has been Caroline Wozniacki, who seems able regularly to defeat any of the others. An interesting wave of potential new challengers has taken shape, at the moment seemingly led by Kleybanova, Wickmayer, and Azarenka, though none of these has been able to match Wozniacki in separating herself above the others.
 
None of the above were particularly successful on clay last year. Listed here were the leaders in ranking points won in singles on clay courts in 2010, along with their foremost clay successes during that year.
 
-- #1. Francesca Schiavone (won Garros, won Barcelona)
-- #2. Samantha Stosur (runner-up at Garros and Stuttgart, won Charleston)
-- #3. Jelena Jankovic (semi-finalist Garros and Italy, quarter-finalist Madrid)
-- #4. Aravane Rezai (won Madrid, Bastl)
-- #5. Venus Williams (runner-up Madrid, quarter-finalist Italy)
-- #6. Caroline Wozniacki (won Ponte Vedra Beach, semi-finalist at Charleston)
 
The top five achievers of 2010, listed above, have not sustained their success. Rezai has failed badly in her results, and Venus Williams faces continuing leg-injury problems, remaining sidelined at Miami. Meawhile Schiavone, Stosur, and Jankovic appear to be healthy, but all three have been playing below their past levels of late, failing to defend their seeded levels at Indian Wells for example.
 
Thus Wozniacki, listed at #6 above, is the leading favorite to succeed in the major clay events ahead. Caroline showed early clay-court success in 2009, when at age 18 she reached the final in Madrid and scored early-round success at Italy and Garros. Set back by an ankle injury incurred in the 2010 Charleston semis, she thereafter scored a remarkable run of success in hard-court play, capturing the #1 ranking for the year. After briefly losing the top ranking, she now holds it firmly after winning Indian Wells 2011. Woziacki's temperament and patient playing style seem ideally suited to winning on clay, but she will have to show that her first and second serves, which are softer than those of most of her leading rivals, can withstand attack by heavy-hitting serve-returners.
 
Several past clay champions remain on the scene. Last year's unexpected winner in Rome, Maria Jose Martinez Sanchez, has not again challenged the leaders and indeed lost to Wozniacki by convincing scores at Indian Wells. Safina, who in 2009 won Rome and Madrid and was runner-up at Garros, seems now to have passed the low point in her descent, but the road back for Dinara yet seems very long.
 
Most threatening to Caroline's possible success are the other two members of the current women's Big Three. Kim Clijsters is returning from her injury and is entered in the draw at Miami. Kim was twice a finalist at Garros early in her career, and she won Rome in 2003. A wonderful mover and aggressive striker, her sometimes-seen impatient manner of play could impede her success in the coming clay ordeals. Meanwhile Vera Zvonareva stands at her career peak, thanks to her long run of strong play on hard courts. She has never done well in the clay events, however, and that was the case in in 2010, though many of her disappointments were traceable to injuries.
 
The men's clay champion for 2011 seems preordained. But another turn in the emerging dominion of Clijsters and Wozniacki among the women seems just ahead, with Serena yet on the sidelines.
 
--Ray Bowers
Arlington, Virginia, U.S.A.