qwads back triceps gucesemus maximus
obviously most of the work is going to come from the arms, shoulders and legs but here is a list of the muscles used THE MOST but are not the only muscles used : biceps brachii, triceps brachii,...
All.
Pretty much all of the main muscles in the legs are used in tennis: the quadriceps, the hamstrings, the gluteus maximus, and the calves.