DOHA (AFP) - Evergreen Serena Williams admitted on Saturday that she never thought she'd still be playing tennis into a third decade as she becomes the oldest woman to take the world top ranking.

"I never thought I would be playing at 31, although I don't feel 31. I don't know where the time went," said the American, the winner of 15 Grand Slam singles titles.

"I never, never, never thought I would still be out here, but I have nothing else to do. I'm pretty good at tennis still, so why not? I'd just rather I didn't sit at home all day." Williams celebrated her recapture of the world number one ranking with her best performance of the year in a 6-3, 6-2 trouncing of Maria Sharapova to reach the Qatar Open final on Saturday.

On Sunday, she will face defending champion Victoria Azarenka, the outgoing world number one, for the title.