It is very normal to be a little nervous in anticipation of a match. This is especially true if the match is important to you. Nervousness generally dissipates soon after the match starts.
If you get so nervous that you become nauseated then you have put too much pressure on yourself. Remember that it is just a game, that you love to play it, and that you are not making your living playing it. Quit worrying about the outcome of a match and concentrate on each point one at a time. Go with what you do best when you are nervous.
For More Information, see: July 1997 Mental Equipment: Stress Relief in Tennis