Have your ever wondered how the professionals and even the better junior players can hit the ball so hard and for so long? I call it "bending the ball"!
 
Most players think their racket is very stiff or rigid. However, what actually happens when you strike the ball with the racket is quite different. First, the ball, which is spherical, does not stay perfectly round. It actually tends to flatten or lose its shape. This forced compression makes the ball more agitated because it likes the original "round" shape. Secondly, the strings also compress and this allows the ball to trampoline back into the racket head. With the new "poly-type" strings this becomes easier than with the older type monofilament strings. This also acts as a catapult or trampoline. Now, the strings are upset that they are not in their usual position and they are anxious to get back to normality. As they try to get back to normal they "shoot" the ball off. And thirdly, the entire racket you bought looks very strong and stiff. However, it is designed to be very bendable and flexible. Once the ball hits, the entire racket flexes and bends, also wanting to return to the original shape.
 
Thus, if you see the whole picture, this is what happens in a millisecond of time. The ball hits the strings and flattens, forcing the strings to trampoline and create more force and this bends the racket back like a trampoline even adding more force. You can see how the modern game has evolved. The balls are livelier, the strings are very powerful and responsive and the new fiberglass racket frames are super light and flexible and very powerful, the perfect storm. There, you can see how the pros can generate such power for so long by "feeding" off of each other's power.
 
The modern game has "morphed" into super power. In the old days, with wooden and metal rackets, this was not possible. The "old game" of chipping, serve and volley, drop shots, lobbing, etc. are over. Make way for "bending the ball."
 
(P.S. If you don't believe this just watch the professional's arm when they are at the top of the serve. You can literally see his or her arm slow down and the racket head will be a blur. He or she is bending the ball.)
 
Good luck on the courts!