Rules

12. When to Serve
The Server shall not serve until the Receiver is ready. If the latter attempts to return the service, he shall be deemed ready. If, however, the Receiver signifies that he is not ready, he may not claim a fault because the ball does not hit the ground within the limits fixed for the service.

USTA Comment: The Server must wait until the Receiver is ready for the second service as well as the first, and if the Receiver claims to be not ready and does not make any effort to return a service, the Server's claim for the point may not be honored even though the service was good. However, the Receiver, having indicated he is ready, may not become unready unless some outside interference takes place.