In short, a robotic tennis partner. A robotically propelled mobile cart that moves along the court's baseline. It uses a distance/location sensor to see where an incoming ball is headed and intercepts it along the baseline (1 axis). A net atop the cart would catch the ball.
Assuming it all works and the cart can move fast enough, etc., a next step/version would be to add a ball machine to the cart. As the ball approaches, another (recently caught) ball would be sent back over the net to the human player, and so on. It would be a like real-life pong game.
I have some ideas for the cart/platform design I'd share with you. This could be broken into several steps. First, a POC: perhaps that a little pre-made robot car with the right sensor can track a tennis ball and move toward it as envisioned (or at least a sensor-based simulation). Next and most importantly, scale up the cart by size and weight and validate it can do the job. Finally, the ball machine would be added.
The tangible short term goal is to have you produce a 4ft x 2ft platform-on-wheels that can move about 12 ft in either direction in response to an incoming tennis ball and intercept about 90% of them.