(CNN) -- An exhausted but elated Serena Williams was left celebrating her 11th title of the season to round off 2013 by claiming the WTA Championships in Istanbul Sunday.
The World No.1 dug deep from a set down and three all in the second against China's Li Na to reel off nine straight games to clinch the prestigious title, 2-6 6-3 6-0.
"I can't believe I won. I was so tired. Honestly, did I really win? Because she played so well," Williams said in her post match interview.
"I'm just overjoyed, to be honest," Williams added. "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 had to grind out a three set semifinal win over Jelena Jankovic Saturday and fell a set down to her fourth seeded Chinese opponent in the final as the tiredness set in.
Williams pulled 3-0 ahead in the second set, only for Li to win three straight games to level.
A lesser player might have crumbled, but the 17-time grand slam champion took her game to another level and finished it off with her 30th winner of the match.
Victory took her prize money for the season to a record $12,385, 572 and at 32 years and one month she is the oldest winner at the end of season championship.
It was her 78th WTA career title and she shows no sign of letting up in her pursuit of records with the first grand slam of the 2014 season in Australia next January firmly in her sights
Li has also finished the season strongly, the first Chinese player to make the WTA Championships final.
She is set to move to her hightest ranking of World No.3 in the new list and showed enough in the opening set and a half to give her hope of further grand slam success to add to her 2011 French Open triumph.