In 2006, the US Open became the first Grand Slam tournament to implement instant replay reviews of calls, using the Hawk-Eye computer system. Available only on the stadium courts (Ashe and Armstrong), each player was allowed two challenges per set plus one additional challenge during a tiebreak but was not penalized with the loss of a challenge if it was upheld. The USTA announced that starting in 2008, each player will be given three challenges per set with an extra challenge if the set goes to a tiebreak.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 millimeters, resulting in an accuracy of 99.2%.
During the 2006 US Open, 30.5% of men's challenges and 35.85% of women's challenges were overturned. During the 2007 US Open, 95 out of 320 challenges were overturned - or 30.6%.