[Allsquared]

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2.6.6 It is the responsibility of the player to serve so that the umpire or the assistant umpire can be satisfied that he or she complies with the requirements of the Laws, and either may decide that a service is incorrect.

2.6.6.1 If either the umpire or the assistant umpire is not sure about the legality of a service he or she may, on the first occasion in a match, interrupt play and warn the server; but any subsequent service by that player or his or her doubles partner which is not clearly legal shall be considered incorrect.