Maria Sharapova has defeated Serena Williams 4-6, 6-2, 6-4 to win the season-ending WTA Tour Championship and underline her status among the sport's elite.

"I'm still shaking. I can't believe it, the way I pulled it out," said 17-year-old Sharapova, who crumpled to her knees after lifting the match in one hour 46 minutes.

Williams, despite being hampered by an abdominal injury, captured the first four games of the third set.

Eventually, Sharapova was able to take advantage of Williams' weakness, claiming a victory that means she will finish the year at No. 4 in the world, after ending 2003 ranked 32nd.