Istanbul, Turkey (SportsNetwork.com) - Serena Williams started slowly, but gained momentum and rallied for a three-set win over Li Na in the final of the season-ending WTA Championships.

Williams battled back for a 2-6, 6-3, 6-0 victory on Sunday to cap one of the greatest seasons of her career. She finished with a match record of 78-4, won 11 titles -- including the French Open and U.S. Open -- and earned more than $12 million in prize money.

The American star claimed the season-ending title for the fourth time overall and for the second straight year. She had already clinched the year-end No. 1 ranking.

"I'm just overjoyed, to be honest," said Williams after the match. "It's really awesome. It's such a special moment. To finish the year No.1 in the world and win this title after 40 years of the WTA, it means even more."

Williams' 11 titles in 2013 are the most for any woman since Martina Hingis won 12 in 1997.

After gutting out a three-set win Saturday against Jelena Jankovic in the semifinals, Williams struggled at the outset on Sunday. Li broke twice early for a 4-1 advantage and quickly closed out the first set.

Li did not face a break point in the opening set and had a break point in the opening game of the second set. Williams, though, fought it off and woke up. She opened a 3-0 lead, but Li battled back to square the set at 3-3.

It was all Williams from there.

The 17-time Grand Slam champion won the next nine games, finishing it with a backhand winner down the line.

"I can't believe I won," Williams said in an on-court interview afterward. "I was so tired. Honestly, did I really win? Because she played so well."

Williams improved to 10-1 lifetime against Li, including 4-0 this year. The two last met in the U.S. Open semifinals, a 6-0, 6-3 Williams rout.

Li, who will vault to No. 3 in the world after this week's run, hasn't beaten Williams since 2008 in Stuttgart.