The medicine ball (medball), a great tool for any tennis fitness program. Medballs are simple yet effective ways to improve strength and power output while using athletic movements. They are also a great way to relieve stress! 
Medballs are essentially just weighted balls used for training. What is great about them, especially for tennis, is that you can mimic motions used during a match and work on your strength, power and endurance. You can do medball exercises on your own or with a partner, if you do it alone try to find a solid wall you can throw it against. 
One of my favorite exercises for tennis players is the Dynamic Forehand/Backhand drill. A solid wall works best for this one. You do a forehand side throw against the wall and let it bounce once back to you, pick it up, and throw a backhand side throw, then repeat. As soon as you let go of the ball you must get yourself ready to catch and throw again from the opposite side so your feet are always moving and your always paying attention. This is a great exercise for your core and building up rotational power for your hits.
Other exercises that involve the medball include; Overhead Slams, Chest Pass, Broad Jump and Throw, Squat Jump and Throw, Overhead Throw, Open/Closed Stance Side Throws, etc... There are many different medball throwing exercises along with variations to all of them. The medball is a very versatile piece of equipment and you can mimic most sport specific movements with it. Add some of these exercises to your current training and I know you'll feel the difference.