In some tournaments, line judges who would be calling the serve, were assisted by electronic sensors that beeped to indicate the serve was out. This system was called "Cyclops". Cyclops has since largely been replaced by the Hawk-Eye system. In professional tournaments using this system, players are allowed three unsuccessful appeals per set, plus one additional appeal in the tie-break to challenge close line calls by means of an electronic review. The US Open, Miami Masters, US Open Series, and World Team Tennis started using this challenge system in 2006 and the Australian Open and Wimbledon introduced the system in 2007. In clay-court matches, such as at the French Open, a call may be questioned by reference to the mark left by the ball's impact on the court surface.
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