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 wildcard to win the tournament.
The Arthur Ashe Kids' Day was held on August 29, 2009, prior to the start of the tournament. It featured an exhibition tennis match involving American player Andy Roddick and Great Britain's Andy Murray, who were joined by actor and comedian Will Ferrell. American Idol winner Jordin Sparks performed her hit single "Battlefield", along with rising stars Honor Society and Justin Bieber. There were also tennis matches and contests featuring Serena Williams, Ana Ivanovic Andy Roddick, Roger Federer, Maria Sharapova, Kim Clijsters, and James Blake. The popular tennis and music festival, which included interactive games, musical entertainment and tennis clinics, was hosted by television personalities Susie Castillo and Quddus.
The first day of the tournaments saw many seeds get through pretty easily in straight sets: Roger Federer, James Blake, Lleyton Hewitt, Nikolay Davydenko and Radek Štěpánek. The day also produced a couple of upsets, when Mikhail Youzhny continued 26th seeded Paul-Henri Mathieu's horrible US Open record by beating him in four. While, tall American John Isner did the same to 28th seeded Victor Hănescu by ending the Romanian's 2009 bid in three with the second set going to a tie-break 16-14. The exciting day continued with a couple of five-set marathons, with Simon Greul defeating Giovanni Lapentti, Guillermo García-López out-lasting Peter Polansky, and Jan Hernych surviving against Rainer Schüttler. The days last match featured America's hope Andy Roddick prevailing over Björn Phau in straight sets in a match that started at 11:00 pm.
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 Radwańska, 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.