I had same wrist pain whenever I played tennis.
Irregardless of whatever causing the problem, the best solution is wraping up with good tape ALL THE TIME when you play tennis.
There will be 3 things can cause: 1. bad technique 2, stiff racquet with poly + high tension 3. previous injury at wrist or ?
I tried wrist exercise with weight but it doesn't seem to help me. Taping is the best way. Rest with ice for 1 week or so until pain disappears then you start to play with taping. You will not have pain again. Maybe you might have some sore after heavy playing day.