Two-time defending U.S. Open champion Kim Clijsters will not go for three in a row this year because of a stomach muscle injury.

Clijsters, who missed Wimbledon with an ankle injury, pulled out of a tournament in Toronto this month with a muscle strain on the left side of her stomach. In a statement Friday, she said "two weeks of rehab is not enough to heal this injury."

"Obviously I'm very disappointed," she said. "I trained very hard this summer and felt in a good shape to play the U.S. Open."

Clijsters, ranked No. 3 in the world, said she also would pull out of tournaments in Japan and China in September.

Her absence leaves Serena and Venus Williams and Maria Sharapova as the headliners on the women's side of the U.S. Open, which begins Aug. 29. Serena won in Toronto but pulled out of this week's event in Cincinnati with a toe injury.

Clijsters won the U.S. Open in 2009 — an unexpected run through the year's final major 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 mom to win a major since Evonne Goolagong Cawley won Wimbledon in 1980.

Last year, Clijsters repeated at Flushing Meadows. She won the Australian Open this year for her fourth Grand Slam title.