Originally Posted by TheCheese
I think the most important thing is to correct for muscle imbalances and to build general strength to prevent injury.
Hitting a monster forehand isn't about being able to lift huge weights. It's about how quickly and efficiently you can complete the kinetic chain.
Bingo. Tennis players needn't have great musculature. What you need to have is the musculature to let you do what you need to. Explosive calves and quads to sprint, flexy hamstrings and ad(ab)ductors for quick lateral movements, strong abdominals and hip flexors to build everything on top of, solid back (especially lower) muscles and traps for stability on serves, etc. Notice I'm not saying larger or being able to move more weight with these muscles. On the contrary. As you already said, simply adding mass just makes you slower. Build these muscle groups using useful movements. Although you might look a bit of a burk, on-court medicine ball training cannot be understated.