Kimiko Date-Krumm (42 y.o.)
She came back in 2008 after nearly 12 years off the tour. Since the comeback, she has won several ITF titles, and then won her eighth WTA Tour title at the 2009 Hansol Korea Open in Seoul, thus becoming the second-oldest player in the Open era, after Billie Jean King, to win a singles title on the WTA Tour.
She is also still a power on the tour, often giving much higher ranked opponents a humiliating defeat.
A champion is afraid of losing. Everyone else is afraid of winning.