Getting nervous before or during a match is completely normal.
I lost many matches as a junior player only because I did not know how to level my state of mind while playing.
There were times when I embarrassed myself by losing to lower-ranked players when everybody who knew me expected me to win easily against them…
But, fortunately, over the years, I have become better at controlling not only my temperament on the court but also my internal emotions.
Here are some techniques that helped me (and still do) before and during tennis matches:
1. Control your breathing – do it slowly and deeply -visualize the air going in and then out.
2. Focus on the strings as soon as the point ends – try to clear your mind and keep your eyes on a particular spot – do not let your eyes shift for at least 10 seconds.
3. Relax your muscles – feel (imagine) your shoulders tensionless.
4. Walk slowly and with confidence.
5. If you have a ritual that makes you feel better (fixing your strings, pulling up your t-shirt sleeves, using the towel, drying your hands, etc.), stay with it.
6. Use positive talk: “I love this sport!”, “This is fun!”, “I enjoy playing!” etc.
7. One way of relaxing is to sing (in a very low voice) or hum your favorite song in between points.
8. Recall a time you played very well and were happy with your game.
You don’t have to do all these techniques at once during the match. Just pick the ones that work for you; I am sure you might already have one that helps you.