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 men's singles final, five time consecutive winner Roger Federer lost to Argentina's Juan Martín del Potro in a match lasting over four hours.