Before The Serve, 2010 US Open

The serve is one of the more difficult shots for a novice, but once mastered it can be a considerable advantage. Advanced players can hit the serve in many different ways and often use it as an offensive weapon to gain an advantage in the point or to win it outright. Because of this, professional players are expected to win most of their service games, and the ability to break an opponent's serve plays a crucial role in a match.
A legal serve occurs when the ball lands in the cross-court service box. If the ball lands outside the box, it is a fault. If two faults of any kind are given in a point, it is a double fault and the point is awarded to the receiver.
A foot fault takes place when the server's feet get into an illegal position while serving. A server's feet must not touch the baseline or center line until the ball is hit. The server is also required to stay roughly on the same position to prevent the opponent from being misled as to where the serve will originate. Running or walking while serving is not allowed.