Two-time defending champion Serena Williams withdrew from the Madrid Open on Friday because of a left thigh injury. The top-ranked American has had her thigh heavily bandaged during play at the clay-court event, and said the injury occurred during a first-round victory over Belinda Bencic on Sunday. It's so frustrating,'' Williams said in a statement. ''This is not the way I wanted this week to end.'' The withdrawal gave 2011 champion Petra Kvitova a walkover victory and a spot in the semifinals. More...