Two-time defending U.S. Open tennis champion Kim Clijsters of Belgium has withdrawn from this year's tournament because of a stomach muscle injury.

Clijsters had missed Wimbledon in June with an ankle injury and pulled out of a tournament in Toronto this month with a muscle strain on the left side of her stomach. In a statement Friday, the 28-year-old said she was very disappointed but that two weeks of rehabilitation was not enough to heal her injury.

The world's third-ranked female said she also would pull out of tournaments in Japan and China in September.

Clijsters' absence leaves American sisters Serena and Venus Williams and Russian Maria Sharapova

as the headliners on the women's side of the U.S. Open, which begins August 29 in New York.

Serena won the Toronto tournament but pulled out of this week's event in Cincinnati with a toe injury.

Clijsters won the U.S. Open in 2009 after a two-year break during which she had a baby girl. She became the first unseeded woman to win the U.S. Open and the first mother to win a major since Australian Evonne Goolagong Cawley won Wimbledon in 1980.

Last year, Clijsters repeated as U.S. Open champ, and she won the Australian Open last January for her fourth career Grand Slam title.