I think it is the pressure you put on your own self that causes you to choke. This happens when we try real hard to "move up the ladder" but when it comes time to put out when it counts - fear sets in. False expectation creeps into our minds and we sort of get a nervous panic. Lots of little demons enter you mind such as "gee, maybe I am not this good" or "maybe I should be on JV and I would win easier", or what will the coach or other players think if I lose".
I dont know what bad thoughts enter your mind before match play but it certainly comes from a lack of inner confidence in your ability. You have to believe in yourself as a person that no matter what happens "win or lose" you are a good man capable of doing well in this life. Tennis is not a game of just strokes. There is an emotional and mental part of this game that needs to be developed and wants to mature. Most athletes stop here.
Instead of focuing on outcomes of matches (which is what causes mental paralysis and fears) focus on what you want to accomplish during the match. Set some performance goals such as "I only want to make two double faults per set".
Nerves will always be there before and sometimes during you match, this is normal, it is energy wanting to be released. Be sure to release the energy in a positive direction for yourself even though you feel crummy at first.
Here are some tips at prematch:
1. Mental toughenss is a LEARNED BEHAVIOR: Mental toughness may appear to be an "inborn" talent, only reserved for the gifted champions. But nothing could be further from the truth. It is a learned behavior. So always, practice with pressure. Simulate points so you become comfortable. Always try to build pressure in your practices. Have your coach help you or your teammate.
2. The Success Formula: top level tennis is 1/3 physical, 1/3 mental & 1/3 emotional. Most players spend 100% of their energy, time and money only on the physical.
3. PRe-match relaxation ritual: Take 10 minutes before each match to calm yourself - deep breathes. Controlling your heart rate is a critical factor in playing in the Zone. (a) turn off all outside stimuli (B) Close your eyes, breath deep & sit still (c) As hard as you can, tighten (for 5 seconds) then relax all muscle groups. Notice the different feelings between tense and relaxed. (d) After relaxing each muscle, repeat "I have the answers to all his/her shots". (e) Before matches, Pros can often be found in a quiet corner of the locker room doing just that!
4. Pre-match visualization ritual: Take the next 10 minutes before the match to rehearse each stroke & favorite patterns. This way you have already been there, in your mind's eye.
5. Proactive ways to relieve anxiety: (a) Exercise: A quicj hit or run is a great short-term solution. (b) Assertive action: conquering your demons is the real long-term goal.
There is more but in a study down by the USTA & David Hemery's Sports Excellence Book, tennis champoins contributed these above rituals as key in helping them overcome their demons and seperating themselves from the average athelete.
Remember nerves will always be there, now that your aware of them, you need a game plan on how to deal with them so you can play.