Though this is her second retirement, Kim Clijsters' career officially ended with a second-round loss to 18-year old Laura Robson at the U.S. Open. The Belgian national was No. 1 in the world in 2003, won four grand slam titles and 41 career titles in her illustrious career. Clijsters first retired in 2007, but returned to the court two years later and won the 2009 U.S. Open, just her third tournament since after returning. Clijsters and fellow Belgian Justine Henin engaged in one of the classic rivalries in women's tennis, meeting 25 times and in three Grand Slam finals. Clijsters finished the rivalry ahead 13-12, but Henin won all of the Grand Slam final matchups. Clijsters was ranked No. 25 before her loss to Robson.