If you wanna get good at tennis you have to use fresh balls. Balls that are flat or fuzzless just dont perform the same as a new ball regardless of brand. Any new ball is better than an old ball.
Using flat balls for drills is fine. If you're going to rally, play points, or play games you have to use a fresh can.
Serves, groundies and the like are much more lively with fresh balls.
"In the 1980's two men dominated--sometimes each other, most of the time everyone else."