Originally, it was scheduled to end with the men's singles final match on Sunday, September 13, but due to rain the tournament was extended by one day. Like the Australian Open, the tournament featured night matches.
Former World No. 1 and 2005 US Open women's singles champion, Kim Clijsters, competed in the 2009 US Open after being granted a wild card entry, returning to professional tennis after more than two years of retirement. She made it to the women's singles semi-finals, where she knocked out the defending champion Serena Williams in controversial circumstances. In the final, Clijsters defeated Caroline Wozniacki, the first Dane, man or woman, to reach a Grand Slam final in the Open Era, in straight sets: 7-5, 6-3. Clijsters thus became the first mother to win a Grand Slam since Evonne Goolagong Cawley in 1980. In the process, she also became the first unseeded player and wild card to win the tournament.
In the men's singles final, five-time defending champion Roger Federer lost to Argentina's Juan Martín del Potro in a match lasting over four hours.
On the women's side, former world no. 1 Kim Clijsters opened the play, winning over Viktoriya Kutuzova losing only two games. Joining her were defending 2009 Wimbledon and Australian Open champion Serena Williams, Amélie Mauresmo, Victoria Azarenka, Flavia Pennetta, Elena Vesnina, Agnieszka Radwanska, Li Na and Marion Bartoli who all won in two sets.
Australian Samantha Stosur was pushed by veteran player Ai Sugiyama in three sets, Daniela Hantuchová got past American Meghann Shaughnessy in three as well. In the first night match, Venus Williams was able to claw her way to avoid an upset to win against Russian Vera Dushevina in a very close three-setter. Venus committed four foot faults in the match. With Venus and Serena getting past the first round, a few American players saw success on the women's side with Vania King, Bethanie Mattek-Sands, and Jill Craybas getting to the second round. Austrian and 28th seeded Sybille Bammer, who defeated Serena Williams in Cincinnati, and 25th seeded Kaia Kanepi were the only casualties of the day.