The players (or teams) start on opposite sides of the net. One player is designated the server, and the opposing player, or in doubles one of the opposing players, is the receiver. Service alternates between the two halves of the court. For each point, the server starts behind his baseline, between the center mark and the sideline. The receiver may start anywhere on their side of the net. When the receiver is ready, the server will serve, although the receiver must play to the pace of the server.
In a legal service, the ball travels over the net (without touching it) and into the diagonally opposite service box.
If the ball hits the net but lands in the service box, this is a let service, which is void, and the server gets to retake that.
If the first service is otherwise faulty in any way, wide, long or not over the net, the serving player has a second attempt at service.
There is also a "foot fault" which occurs when a player's foot touches the baseline or an extension of the center mark before the ball is hit. If the second service is also faulty, this is a double fault and the receiver wins the point. However, if the serve is in then it is considered a legal service.
A legal service starts a rally, in which the players alternate hitting the ball across the net.
A legal return consists of the player or team hitting the ball exactly once before it has bounced twice or hit any fixtures except the net provided that it still falls in the server's court. It then travels back over the net and bounces in the court on the opposite side.
The first player or team to fail to make a legal return loses the point.
No-ad: The first player or doubles team to four points wins the game. One side does not have to win by two points. When the game score reaches deuce, the receiving player has the option to choose on which side of court they want to receive for the final game-deciding point.
Pro set: Instead of playing multiple sets, players may play one "pro set". A pro set is first to 8 (or 10) games by a margin of two games, instead of first to 6. A 12-point tiebreaker is usually played when the score is 8-8 (or 10-10). These are often played with no-ad scoring.
Super tie-break: This is sometimes played instead of a third set. This is played like a regular tie-break, but the winner must win ten points instead of seven. Super tie-breaks are used on the ATP and WTA tours for doubles and as a player's choice in USTA league play.
In serious play, there is an officiating head judge or chair umpire (usually referred to as the umpire), who sits in a raised chair to one side of the court. The umpire has absolute authority to make factual determinations of tennis rules.
The umpire may be assisted by line judges, who determine whether the ball has landed within the required part of the court and who also call foot faults.
There also may be a net judge who determines whether the ball has touched the net during service. In some tournaments, certain line judges, usually those who would be calling the serve, are replaced by electronic sensors that beep when an out call would have been made.
In some open-tournament matches, players are allowed to challenge a limited number of close calls by means of instant replay. The U.S. Open, the Miami Masters, U.S. Open Series, and World Team Tennis started using a "challenge" system in 2006 and the Australian Open and Wimbledon introduced the system in 2007. This used the Hawk-Eye system and the rules were similar to those used in the NFL, where a player gets a limited number of instant-replay challenges per match/set. In clay-court matches, a call may be questioned by reference to the mark left by the ball's impact on the court surface.
The referee, who is usually located off the court, is the final authority about tennis rules. When called to the court by a player or team captain, the referee may overrule the umpire's decision if the tennis rules were violated (question of law) but may not change the umpire's decision on a question of fact. If, however, the referee is on the court during play, the referee may overrule the umpire's decision.
Ball boys may be employed to retrieve balls, pass them to the players, and hand players their towels. They have no adjudicative role. In rare events (e.g., if they are hurt or if they have caused a hindrance), the umpire may ask them for a statement of what actually happened. The umpire may consider their statements when making a decision. In some leagues, especially junior leagues, players make their own calls, trusting each other to be honest. This is the case for many school and university level matches. However, the referee or referee's assistant can be called on court at a player's request, and the referee or assistant may change a player's call. In unofficiated matches, a ball is out only if the player entitled to make the call is sure that the ball is out.