Tennis requires a lot of quick lateral foot movements, so exercises that mimic this will help improve your speed and power in the lateral direction.
Some examples are:
- Lateral shuffle from side to side - add an ankle resistance tube for more challenge
- 'Cone hops': think speed skater - choose two objects and your goal is to push off your outside foot to hop all the way to the other side, land and repeat
- Plank jumping jacks - in a push up position on the floor, keep your hands and upper body still as you perform a jumping jack movement with your feet