Here is a tennis robot that helps you practice tennis without a partner. The Spinshot-Player Tennis Ball Machine lets you use your phone to program speed, spin, height, angle, and feed rate for each shot. The machine comes with 12 pre-programmed drills. It measures only 18″x12″x20″ and weights of only 18kg.
The Spinshot Player tennis robot has a metal design. You can find out more about it here.