Ok, now that we have weeded out the ridiculous posts.
OP, fully developed legs should be at the core of your training. All your power on serves, ground strokes, and volleys should come from your legs and core. Squats, Leg presses, and lunges are great for gaining mass and strength on your legs. Look at Nadal, Berdych, and Murray they all have huge quads.
Throw in an ab workout every other day and you should be crushing the ball in no time.
One of my favorite quotes from Agassi's book:
Gil Reyes " Strong legs obey, weak legs command."
Alright, well, the reason you want strong legs and a strong core is so you can cover the court at a high level for the entire match. You have to be able to sprint, stop, change directions, jump, squat, lunge, and all sorts of stuff. All of these activities benefit from strong legs and core.
You probably get a little bit of power from leg drive or core rotation, but I'm inclined to think that the majority of the power comes from the racquet colliding with the incoming tennis ball. An energy transfer occurs, and as long as the racquet impart more energy to the ball than the other way around, the ball is propelled forward with a new velocity, trajectory, and spin profile.
Sure, maybe some of this comes from the legs. But that's not why you need a strong lower body to play tennis well.
It's a fact, 65% of your power comes solely from arm swing.
The other 35% is core, legs, all that kinetic crap.
I'm inclined to agree with the TT GOAT, LeeD, although I really wish he'd source his work. :twisted:
OP - there is no weightlifting exercise that has a direct relationship with topspin. Topspin is the effect of technique, not necessarily muscle strength.
You mentioned that you felt your form was being compromised while weight training. From what I understand, "strength training" (heavy weights) is not recommended while you're "in season." But what the hell do I know? I'm not a trainer. But as your muscles grow you do have to adjust how to handle the new body mass. Stretching / flexibility and training on the court are essential to maintaining form. Just know that proper weight training
requires rest.