“Two weeks of rehab was not enough to heal this injury,” Kim Clijsters said in a statement Friday annoucing her withdrawal from the U.S. Open. “Obviously I’m very disappointed.” (Darren Calabrese/AP)

The U.S. Open has lost three-time (and two-time defending) champion Kim Clijsters, who informed tournament officials Friday that she’d be unable to take part in the season’s final major because of a strained abdominal muscle.

Clijsters, 28, ranked No. 3 in the world, suffered the injury earlier this month and missed a tournament in Toronto as a result. She said at the time that she expected to be able to defend her U.S. Open title. But the injury, similar to the one that kept Andy Roddick out of this month’s Legg Mason Tennis Classic, proved more problematic.

“Two weeks of rehab was not enough to heal this injury,” Clijsters said in a statement released by the U.S. Tennis Association. “Obviously I’m very disappointed. I trained very hard this summer and felt in good shape to play.”

Clijsters also withdrew from upcoming tournaments in Tokyo and Beijing, saying that she risked a relapse if she returned before fully healed.

With Clijsters out, the favorites for the women’s title when the U.S. Open gets underway Aug. 29 are three-time champion Serena Williams and 2006 champion Maria Sharapova, both of whom have rebounded impressively from extended layoffs.

Clijsters claimed her first major at the U.S. Open, winning the title in 2005. Her victory in 2009 was particularly impressive, coming within months of her return to competition after taking two years off to marry and start a family. Clijsters’s daughter, Jada, joined her mother for the on-court trophy presentation that followed. Mother and daughter accepted last year’s trophy together, as well.