Which bounces higher a new tennis ball or an old tennis ball?

Answer 1

It has been demonstrated that an "old tennis ball" actually has a higher coefficient of restitution than a "new tennis ball". That means that if the two balls, hit the same object at exactly the same velocity, the "old ball" would rebound further. I know this seems counter-intuitive, but it's true. Try it some time.

The reason a tennis player thinks that an old ball is "dead" is simply because it reacts in a sloppy manner off the racket. It behaves unpredictably. Experienced players are not used to playing with old balls. But that does not mean that it has less bounce or cannot be hit as hard. Quite the opposite.