Kim Clijsters pulls out of U.S. Open with an abdominal injury . Two-time defending champion Kim Clijsters has pulled out of this year 's U.S. Open with a stomach muscle 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 .
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 .