Before The Serve, 2010 US Open

A serve (or, more formally, a service) in tennis is a shot to start a point. A player begins a serve by tossing the ball into the air and hitting it (usually near the highest point of the toss) into the diagonally opposite service box without being stopped by the net. The ball can only touch the net on a return and will be considered good if it falls on the opposite side. If the ball contacts the net on the serve but then proceeds to the proper service box, it is called a let; this is not a legal serve in the major tours although it is also not a fault. Players typically serve overhead, but serving underhand, although rare, is allowed. The serve is the only shot a player can take their time to set up instead of having to react to an opponent's shot.
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.