The US Open men's singles championship is an annual tennis tournament that is part of the US Open and was established in 1881. It is played on outdoor hard courts at the USTA Billie Jean King National Tennis Center in Flushing Meadows – Corona Park, New York City, United States. The US Open is played during a two-week period in late August and early September, and has been chronologically the last of the four Grand Slam tournaments of the tennis season since 1987. Newport (1881–1914), Forest Hills (1915–1920, 1924–1977), and Philadelphia (1921–1923) held the event before it settled in 1978 at the USTA National Tennis Center, now the USTA Billie Jean King National Tennis Center, in New York City. The inaugural tournament, in 1881, was reserved for United States National Lawn Tennis Association (USNLTA) club members, before the championships opened to international competitors in 1882. The USTA is the national body that organizes this event. History The men's singles' rules have undergone several changes since the first edition. From 1884 to 1911, the event started with a knockout phase, the All-Comers singles, whose winner faced the defending champion in a challenge round. The All-Comers winner was awarded the title six times (1888, 1893, 1898, 1901, 1904, 1907) in the absence of the previous year's champion. The challenge round system was abolished with the 1912 edition. From 1908 to 1914, when the championships were held at Newport, men's singles draws had exceeded 128 players, but when the event moved to Forest Hills in 1915 entries would be "submitted by clubs, thereby making the clubs weed out mediocre players. Furthermore, the entry fee would be set high enough to prevent cheapskates from entering just to get a seat at the tournament at a lower price and then defaulting". Since 1881, all matches but the All-Comers final and the challenge round were played as the best-of-three sets, before the event switched to best-of-five for all rounds in 1886. Best-two-out-of-three-sets matches were reintroduced for early rounds in 1917, from 1943 to 1945, and from 1975 to 1978. Before 1884, the winner of the next game at five-games–all took the set in every match except the All-Comers final and the challenge round, which were won by the player who had at least six games and at least two games more than his opponent. This advantage format was introduced for the final sets of early rounds