Serena Williams had to recently bow out of the WTA Championships due to a stomach injury she most likely suffered while playing against (coincidentally) her sister Venus. According to Serena, the pain would follow her even when she was engaged in the relatively motionless act of sleeping. Also pulling out of the WTA Championships was Ana Ivanovic due to a viral illness. Goto tennis.com and learn more about the rest of the players in contention for the WTA Championship.