Wimbledon champion Maria Sharapova fought back from 4-0 down in the final set of the season-ending WTA Tour Championships to beat former world number one Serena Williams in Los Angeles.

The Russian 17-year-old clinched a 4-6 6-2 6-4 victory by winning the last six games, capitalising on Williams' abdominal strain to claim the US$1million (£541,000) first prize.

"It's been an amazing week, it's been an amazing year," said Sharapova, who will end the year with a career-best number four ranking.

"This has been an extraordinary experience for me. I finished it off and I'm so happy."

It was Sharapova's second victory over her American opponent this year having overcome Williams in the Wimbledon final in July.

Williams grabbed the opening set in her best start of the tournament but Sharapova took the second set by utilising her powerful ground strokes.

The American appeared to injure herself after dropping her serve in the final game of the second set and she was attended to by the trainer.

Despite being unable to serve effectively the six-time Grand Slam winner raced into to a 4-0 lead before the effects of the injury began to show - Williams reduced to hitting soft serves at only 80mph.

Sharapova regrouped and gradually picked off Williams, levelling the scores at 4-4 when Williams double-faulted a 68mph second serve on break point.

After holding serve to love Sharapova led 15-40 before hammering a forehand service return winner down the line and falling to the court in disbelief.