The Rules of Tennis
The Singles Game
8. Foot Fault
The Server shall throughout the delivery of the Service: a. Not change his position by walking or running. The Server shall not by slight movement of the feet which do not materially affect the location originally taken up by him, be deemed "to change his position by walking or running".
b. Not touch, with either foot, any area other than that behind the base-line within the imaginary extensions of the centre-mark and side-lines.
USTA Comment: The key to understanding this rule is to realize that the Server's feet must be at rest immediately before beginning to serve. Immediately thereafter, the delivery of the service begins with any arm or racket motion, and ends when the racket contacts the ball (or misses the ball in attempt to strike it).
If either foot touches the Court, including the baseline, or the imaginary extension of a line specified in Rule 8b. after his feet are at rest but before he strikes the ball, he has committed a foot fault.
There can be no foot fault if the Server does not attempt to strike at the ball. As long as the Server makes no attempt to strike at the ball, it is immaterial whether he catches it in his hand or his racket or lets it drop to the ground.
USTA Comment: This rule covers the most decisive stroke in the game, and there is no justification for its not being obeyed by players and enforced by officials. No official has the right to instruct any umpire to disregard violations of it. In a non-officiated match, the Receiver, or his partner, may call foot faults after all efforts (appeal to the server, request for an umpire, etc.) have failed and the foot faulting is so flagrant as to be clearly perceptible from the Receiver's side. It is improper for any official to warn a player that he is in danger of having a foot fault called on him. On the other hand, if a player asks for an explanation of how he foot faulted, either the Line Umpire or the Chair Umpire should give him that information.
That pretty well settles it. Thanks for providing that. Maybe the whiners will get off their wayward “Poor Serena” bandwagon now, or at least STFU with their claim that she got a raw deal.