Serena Williams has advanced to face big sister Venus in the final of the Sony Ericsson Championships after Caroline Wozniacki retired with an apparent abdominal injury when trailing 6-4, 0-1. Wozniacki received medical treatment on her stomach muscles in the first set and decided to quit after breaking Williams to start the second. The 19-year-old Dane has been struggling with injuries all week, and said Friday that her aching abdominal muscles were affecting her serve.