Wimbledon champion Maria Sharapova capped her stunning year by overcoming an injured Serena Williams 4-6, 6-2, 6-4 in the final of the season-ending WTA Tour Championships in Los Angeles on Monday.

"I'm still in shock. I can't believe the way I pulled it out," said the 17-year-old Russian, who fell to the ground and screamed with joy at the end of a contest lasting one hour 46 minutes.

• Images from the match

"It's very unreal the way I stuck in there. It's been an amazing year."

After falling behind 4-0 in the third set, the teenager reeled off six games in a row to overpower a hobbling Williams.

The American took a medical timeout for treatment on an abdominal strain after the seventh game of the second set.

After returning to the court 5-2 down, Williams lost about 40 mph from her service speed and had trouble moving to her right.

Williams said she felt the injury come on in the opening game of the match.

"I really thought it was just a stitch, and it would go away," Serena said. "I thought at times I wouldn't finish, but I like to fight. On a 0-10 scale, the pain was a 10 and a half."

"It's extremely disappointing. I figured I had a good chance at this title."

Serena recalled the 2003 Wimbledon semi-final when her sister Venus continued to play against Kim Clijsters despite an abdominal strain and eventually tore it in the match. Venus did play the final where she lost to Serena, but she was then off the tour for the next six months.

"I was thinking I'm not going to be out six months," Serena said. "I wasn't going to go for any big serves because it's not worth it with the New Year coming around."

Sharapova easily closed out the second set but, in the third, Williams began to go for broke off the ground and cracked winners to all angles of the court to storm into a 4-0 lead.

WILTING WILLIAMS

The young Russian eventually took a foothold in the final set when she held serve at 4-0 down and, with Williams wilting, hit back to seal a victory which earned her $1 million.

"She was hitting as hard as she could and everything was going in," Sharapova said.

"She was tough. I couldn't capitalize on the weak serves. I was just trying to find a little opening. I was just mentally strong."

Sharapova won four titles coming into the event, but had losing records against three players she beat, fellow Russians Vera Zvonareva, Svetlana Kuznetsova and Anastasia Myskina. Then she toppled Williams in her home town.

"It shows that I've come a long way in a short period of time," Sharapova said.

"To be able to beat top players and playing five days in a row with my body, it's not very easy. When I was playing against (Amelie) Mauresmo, I was absolutely dead and I didn't think I could go on. I found an inner strength." Sharapova suffered her only defeat to the Frenchwoman in the group stage.

Sharapova, who now moves to a career-high number four in the world rankings, also became the first champion from Russia and only the second player to win the title on her debut in the event. Serena Williams won it on her first appearance in 2001.

Williams said the injury might be an abdominal tear and she plans to have an MRI scan on Tuesday.