<P> In the event of a rain delay, darkness or other external conditions halting play, the match is resumed at a later time, with the same score as at the time of the delay, and the players at the same end of the court when rain halted play, or at the same position (north or south) if play is resumed on a different court . </P> <P> 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 . 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 . However, in ITF tournaments like Fed Cup, the balls are changed in a 9--11 style . 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 . </P> <P> A recent rule change is to allow coaching on court on a limited basis during a match . This has been introduced in women's tennis for WTA Tour events in 2009 and allows the player to request her coach once per set . </P> <P> A competent tennis player has eight basic shots in his or her repertoire: the serve, forehand, backhand, volley, half - volley, overhead smash, drop shot, and lob . </P>

In which year was the on court coaching introduced