Everyone gets nervous.
The best tennis players have methods that they use to keep nerves from affecting their ability to play the next point the way they would like. Most players have a speed that they normally play the point—they either take the majority of the 25 seconds allowed between points, or they play relatively quickly and step up to the line as soon as they are ready. When they start feeling a bit tight, they try to keep the same speed.
Also, the best players have a normal thought process that they go through before playing the next point. It basically involves moving on from the point that was just played, reminding themselves of their general strategy, and choosing one important thought for either the serve or the return of serve (toss the ball out in front, return deep and down the middle, or serve more to the middle of the service box are all common ideas).
They do this on all of the points. During the point, they are playing more by reaction and instinct than anything else. They try to hit their normal shots in the most natural direction (for them) and let things happen. Most of us are not comfortable with the process I just described. We tend to be extra careful or very aggressive when nerves start creeping in. We are very conscious of the nerves and it makes us change what we normally do.
Try taking a few deep breaths, going through something close to what I have described here, and then just play your normal point. You will not win all of these points, but it is likely that you will surprise yourself with how much better you will play when the nerves kick in, and you will give yourself a much higher chance for success.
Steve Annacone, USPTA Elite Pro, is the Director of Annacone Tennis, www.annaconetennis.com and MyHamptonsPro, www.myhamptonspro.com throughout the Hamptons, NY. In addition, Steve and Miguel Coelho have introduced the JET (Junior Elite Tennis) program at the Tucson Jewish Community Center (Tucson, AZ) for high level players ages 8-18. Please contact Steve at firstname.lastname@example.org.