Originally Posted by Cheetah
Grip strength has nothing to do with it. Most of these guys have extremely loose grips.
Maybe, but I can tell you with certainty that the thumb is crucial to the ATP style. Try hitting like Fed or Nadal while keeping your thumb off the handle entirely. Then wind up like Sharapova. You'll find that thumbs aren't necessary to hit like Sharapova but essential to hit like Fed or Nadal.