Instant replay
In 2006, the US Open introduced instant replay reviews of line calls, using the Hawk-Eye computer system. It was the first Grand Slam tournament to use the system. The Open felt the need to implement the system because of the controversial quarterfinal match at the 2004 US Open between Serena Williams and Jennifer Capriati, where important line calls went against Williams. Instant replay was available only on the Arthur Ashe Stadium and Louis Armstrong Stadium courts through the 2008 tournament. In 2009, it became available on the Grandstand court. Starting in 2018, all competition courts are outfitted with Hawk-Eye and all matches in the main draws (Men's and Women's Singles and Doubles) follow the same procedure each player is allowed 3 incorrect challenges per set, with one more being allowed in a tiebreak.