( CNN ) -- Two-time defending champion Kim Clijsters has pulled out of this year 's U.S. Open with a stomach muscle injury . The 28-year-old Belgian suffered the injury at the Rogers Cup tournament in Toronto last week and told the official WTA website that she would not be able to recover in time for the final grand slam of the season which starts on August 29 in New York . `` Two weeks of rehab was 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 , the world number three , has suffered an injury-hit year since claiming the Australian Open title back in January , her fourth grand slam crown . She missed most of the clay court season and then Wimbledon with an ankle injury and was hoping for better luck as she chased a fourth triumph at Flushing Meadows where she won in 2005 , 2009 and 2010 . The injury will also affect her end of season campaign and she acknowledged that she will need careful rehabilitation . `` 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 . '' Clijsters ' withdrawal is likely to leave Serena Williams as a warm favorite to add to her grand slam tally , but the American was herself forced out of the latest WTA event in Cincinnati through injury , but it is not thought serious . Meanwhile , Wimbledon runner-up Maria Sharapova eased into the semifinals of the tournament in Ohio Friday with a straight-sets victory over Samantha Stosur . Fourth seed Sharapova beat the 10th-seeded Australian 6-3 6-2 and next faces second seed Vera Zvonareva of Russia , who battled past unseeded Slovakian Daniela Hantuchova 6-3 7-6 . Ninth seed Andrea Petkovic also reached the semifinals with a 7-5 6-1 win over Nadia Petrova . Petkovic will face 13th seed Jelena Jankovic , who went through when Shuai Peng withdrew before the start of their match with a hip injury .
Kim Clijsters pulls out of U.S. Open with an abdominal injury . Two-time defending champion Clijsters has missed final three grand slams of 2011 . The Dutch star was forced to withdraw from the Australian Open, where she had won her first major title in nearly two years and is now ranked No. 1. Maria Sharapova marches into semifinals in Cincinnati to play Vera Zvonareva .