Because tennis is a game where you have to react to someone else it is important to put some unpredictability training into your fitness and footwork drills.
Tennis is a game where you must SEE a ball, REACT to this ball then EXPLODE over a very short distance. Remember the furthest you will ever move to a ball is 14 meters. Great unpredictable drills for tennis is have a partner (this makes it unpredictable) throw one of 2 tennis balls so as you catch one and throw it back the next is thrown immediately.
The ball can be caught as a volley, on the first bounce, rolled along the ground, caught above the head, dropped so you run like a drop shot and thrown over your head like you run back for a lob.
This is great before tournament play as it gets you moving and warming up eyes, feet and muscles and is tennis specific.