Barbell Squats are perhaps the most effective exercise at working your lower body. As you probably know, tennis relies heavily on lower body strength to zip you around the court during a strenuous match. Focus on going down slowly and exploding upwards. Do not allow your knees to buckle in, as this is just asking for injury.
Dumbbell lunges are a great way to work your lower body, especially your glutes and hamstrings. These muscles are vital in a sport like tennis. Focus on slowly lowering yourself before coming up in a nice, controlled manner. To really work these muscles, go as heavy as you can with the weights, while still maintaining your form.
Reverse dumbbell flys are fantastic for building up the muscles of the rear delt and upper back. These are the muscles that are utilized when you strike the tennis ball with your racket. The more muscular strength you have in this area, the more power you will develop on your shot. Do these sitting down on a bench with your head tucked in. Raise your arms as high as possible, then slowly lower them.
Sprints are a great way to get and/or stay in tennis shape. Lay out some cones, or markers where the red dots are on the court. Sprint forward during the red line portions, and sprint as fast as you can backwards during the yellow line portions. Do this sprint routine for a total of 3 minutes, then rest for 2, then repeat 2 more times.