Kim Clijsters pulls out of U.S. Open with an abdominal injury . Two - time defending champion Williams has reached final four grand slams of tennis . Clijsters sustained the injury in the Rogers Cup in Toronto last week . Maria Sharapova advances 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 . Clijsters sustained the injury in the Rogers Cup in Toronto last week . Maria Sharapova marches into semifinals in Cincinnati to play Vera Zvonareva .