When Serena Williams first appeared on the tennis scene, nearly everyone saw her as the little sister of Venus. But it didn't take long for the younger Williams to prove herself to be a worthy competitor in her own right. Serena quickly began winning—and then winning some more. Some of her most treasured victories have been won alongside her sister; others have been fought against her. But through it all, Serena has never lost her desire to keep competing. And even as the oldest female player to be ranked number one, she still isn't ready to lay down her racket.