As a tennis player and fan myself, this has always been a question that I have wondered about. I know that an ace is when a serve that is in the box goes through untouched. Is a service winner a serve that is hit but not returned, or is it something I have just made up?
If the receiver does not return the ball in play, it can be classified as one of the following: Unforced Error: The serve was not fast and the returner tried to hit a winner but the ball was not in play. One typical scenario is when the receiver clearly tries for a forehand while the serve was directed to the backhand. Forced Error: The serve had enough pace to make the return difficult, the return was somewhat controlled but still went out or into the net. Service Winner: The receiver touched the ball, but he/she had no control whatsoever. The ball went to the stands or hit the ground from the receiver's racket.