Deuce and Advantage

When a game reaches 40-40, standard scoring requires a player to win by a margin of two points rather than simply the next point. Winning the point immediately after deuce gives that player "advantage" — informally, "ad in" if the server has the advantage or "ad out" if the returner does. If the player with the advantage wins the following point, they win the game; if they lose it, the score returns to deuce and the sequence repeats until someone wins two points in a row. This is what allows a single game to theoretically extend indefinitely, and long deuce battles of many advantages back and forth do occur, though they are relatively rare.

The deuce-and-advantage structure is one of the defining features that separates traditional tennis scoring from no-ad formats, and it rewards players who can perform well under repeated close-game pressure rather than settling a game on a single point. Momentum within a deuce sequence is often discussed by commentators and coaches because a player who has just saved several advantage points may carry a psychological edge into the next one, even though each point technically starts even.