Originally Posted by UCSF2012
Hitting flat is the way to get good fast. Topspin is high error. You're swinging away at all your shots, so you miss your targets more often, and you mishit more often. Take controlled swing at 60-70% and hit flat. Hit it 2 ft over the net, but don't hit it so hard it goes out. It's about restraint. Don't believe me? Sampras hit flat, and so does Del Potro.
Unless you hit incredible angles, good players will crush those 70% flat balls. You might as well be feeding it to them with a ball machine. There is a reason that the game has evolved from flat groundstrokes to heavy topspin. And it isn't because it is high error.
That being said, for many people hitting flatter is probably a good idea. It is more efficient so older players or those with lower RHS may not be able to keep a heavy topspin ball deep over the course of 2-3 sets.