Two-time defending champion Kim Clijsters has withdrawn from the US Open after failing to recover from a stomach muscle injury.
It has been an injury-hit season for the world number three, who missed most of the clay-court season and Wimbledon with an ankle injury before being forced out of last week's tournament in Toronto with the stomach problem.
Clijsters said: "Unfortunately I will not be able to defend my US Open title this year because of a stomach muscle injury. Two weeks of rehab was not enough to heal this injury.
"Obviously I'm very disappointed. I trained very hard this summer and felt in a good shape to play the US Open."
Clijsters revealed she has also pulled out of the WTA Tour Premier events in Tokyo and Beijing at the end of September and start of October, and it is certainly a possibility that she will not play again this year.
The Belgian added: "Now there is nothing else I can do than to rest and have treatment every day.
"If a gradual approach is not taken, I will relapse in the same injury, and therefore I have also no other possibility than to withdraw from the tournaments in Tokyo and Beijing."
The US Open was the stage on which Clijsters announced her return to the game after two and a half years in retirement in spectacular fashion in 2009.
Only a month after returning to the WTA Tour, she beat both Venus and Serena Williams, the latter in a semi-final that became famous for Williams' abuse of a lineswoman, before defeating Caroline Wozniacki to secure her second grand slam title.
She then successfully defended the crown 12 months ago, this time beating Vera Zvonareva in the final, before going on to win her first grand slam title outside America at the Australian Open this January.
Clijsters reclaimed the world number one ranking and looked set to be the player to beat in 2011 before injury struck in a most unusual way, the 28-year-old sustaining a serious ankle injury while dancing at a cousin's wedding.
She returned to action at the French Open but was clearly rusty and lost to little-known Dutchwoman Arantxa Rus in the second round.
A recurrence of the ankle problem forced her out of Wimbledon and she did not play again until the Rogers Cup in Toronto last week, where she lasted only 12 games against Zheng Jie before being forced to retire.
US Open tournament director Jim Curley said: "It is unfortunate that Kim has not recovered from her injury in time to defend her US Open title.
"We wish her a full and speedy recovery and look forward to her return to New York next year."