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?
You are right. A service winner is a serve that the receiver hits without making a legal return.
The serve, called an
ace if the opponent cannot get his racket on the ball and a
service winner if the opponent reaches it but cannot play it back to opponent's court. In both situations we are talking about the case when the serve goes to the service box of course