Due to the shakes? Lol?
I think most players, even good ones, will feel nervousness and/or anxiety... I mean the pros talk about it all the time, how they still get nervous, it just means you really care. I still get nervous when I'm playing at the concert or in front of a crowd in general. Really, the only thing one can improve on is how quickly they lose their nervousness or just being naturally very confident with themselves.
Personally, for tennis, I kind of try to wear myself a little during the warm-up with the other player. I always feel my nervousness stems from having a bit too much energy, so when I feel any slight bit of tiredness it seems I calm down a little bit. Sometimes, it doesn't always work that way and especially if the match is a final, I continue to feel a little anxious, however, I do continue to wear myself down a little bit. Obviously I don't do it to the point where I can't run or hit the ball, but just getting rid of that excess energy. Luckily though, I have a adequate serve, so I'm able to hold my early service games without being too nervous about rallying.