Re: Good gym program for tennis?
It hasn't been mentioned here yet, but if you're playing competitive tennis then your fitness/strength training needs to line up in terms of the time of the season you're in. All strength training for tennis should move from muscular endurance to strength/hypertrophy training to power training/plyometrics (nearer to competition). Assuming you've covered muscular endurance as a general player you can move into strength training 8-12 reps (3-5 sets) and then work towards power/plyometrics (2-6 reps) nearer towards competition for your explosive court speed and shots.
This is obviously just a quick outline, have a look at some strength based training that moves into power training/plyometrics and you'll do well. Make sure you focus towards compound exercises (multiple body parts) rather than isolating one body part, since tennis requires full body movements. Also work your core extensively, it's so important to be rock solid as a player.