I have a question. How can playing tennis (which posture) hurt the low back? What exercise can prevent the injury?
You can protect your lower back, and you knees, by making sure that your knees are always bent, this way, when you turn your shoulders to prepare to hit, you hips, which are a ball-and-socket joint, absorb all of the stress, as opposed to L 3/4/5, and your knees, which are not designed to be twisted, but to be only moved in one direction.
Do squats, with or without resistance to train for getting low.