Tennis
Ball changes

Balls wear out quickly in serious play and, therefore, in ATP and WTA tournaments, they are changed after every nine games with the first change occurring after only seven games, because the first set of balls is also used for the pre-match warm-up. In ITF tournaments like Fed Cup, the balls are changed after every eleven games (rather than nine) with the first change occurring after only nine games (instead of seven). An exception is that a ball change may not take place at the beginning of a tiebreaker, in which case the ball change is delayed until the beginning of the second game of the next set. As a courtesy to the receiver, the server will often signal to the receiver before the first serve of the game in which new balls are used as a reminder that they are using new balls. Continuity of the balls' condition is considered part of the game, so if a re-warm-up is required after an extended break in play (usually due to rain), then the re-warm-up is done using a separate set of balls, and use of the match balls is resumed only when play resumes.
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