Female Tennis Player, US Open 2011

Player challenges of line calls
In 2006, the US Open was changed to implement instant replay reviews of calls, using the Hawk-Eye computer system. Each player was allowed three challenges per set plus one additional challenge during a tiebreak. The player keeps all existing challenges if the challenge is successful. If the challenge is unsuccessful and the original ruling is upheld, the player loses a challenge. Instant replay was initially available only on the stadium courts (Ashe and Armstrong), until it became available on the Grandstand in 2009.
Once a challenge is made, the official review (a 3-D computer simulation based on multiple high-speed video cameras) is shown to the players, umpires, and audience on the stadium video boards and to the television audience at the same time. The system is said to be accurate to within five millimetres.