Serve Corrections
Increase the number of practice repetitions.
Stand inside the baseline to serve during practice sessions.
Keep your wrist from unnecessary wobbling.
Reduce the velocity of the serve.
Take the racket to a position behind your back and touch the middle of your back with the top edge of your racket. Lift the racket head a few inches, or centimeters, and make this your starting point. Swing upward at the ball. The serve motion goes up before it goes forward. Don’t let your elbow lead the stroke; keep it up until after the hit. When you hit, reach as high and as far out as you comfortably can. Your arm and racket should be fully extended when contact is made. With the hit, “flop” (pronate) your wrist. Turn it down and away on contact. After contact, continue to bring the racket forward toward the net and then down and across.
The swing to serve a tennis ball is not exactly like the throwing motion, but it has many things in common with throwing a baseball or softball. If you can throw well, you will probably serve well. To assess your throwing ability, try the ready, aim, throw drill.
Tennis: Steps to Success Jim Brown,Camille Soulier