Serena Williams dug deep to secure her fourth WTA Championship title, coming from a set down to beat Li Na 2-6 6-3 6-0 on Sunday.
The world #1 claimed she was running on empty following her semi-final victory over Jelena Jankovic in Istanbul and Na looked set to cash in as she took the first set.
But Williams fought back to win the second and blew her Chinese opponent away in the decider, becoming only the fourth player to win the season-ending event four or more times.
In the first WTA final between two players over 30, Li made a confident start to take the opening set against Williams, who was showing signs of fatigue. The world #3 from China raced ahead and secured the first set in Istanbul on her fourth set point.
But Williams, 32, levelled the match and clinched her 11th title of the year in 2h 9m.
The American won a key game at the start of the second that went to nine deuces and lasted nearly 12 minutes, but Li levelled at 3-3 with her only ace of the match. However, a double-fault and a forehand error gave Williams a chance to serve out the set, with three set points required to close it out.
Another double-fault by Li gave Williams a break of serve at the start of the decider and the 17-time Grand Slam winner swiftly completed her 10th win in 11 matches against the Chinese star.
The triumph marked her fourth WTA Championships success and she became the first player to defend the title since Belgium's Justine Henin (2007).
Williams also became the the eighth female player to win at least 11 titles (which is what former world #1 Ana Ivanovic has won in her career, thus far) or more in a year and the first since Martina Hingis claimed 12 in 1997.
The American, who has now won 78 of 82 matches this year, was confirmed as the year-end world #1 for a third time (2002, 2009) well ahead of the tournament.