Williams wins in Istanbul
Serena Williams summoned up the energy to fight back from a set down to defeat Li Na and cap a remarkable season by retaining her WTA Championships title.
The American described herself as having hit a wall after a strange performance in beating Jelena Jankovic in the semi-finals, and first-time finalist Li dominated the first set.
But Williams found her game in the second and then romped through the decider to win 2-6 6-3 6-0 after two hours and nine minutes.
The match was watched by a record crowd of 16,457, the highest in the three years the season-ending tournament has been held at Istanbul's Sinan Erdem Dome.
The trophy is the 11th Williams has collected in a year that has earned her more than 12 million US dollars - only Novak Djokovic has ever won more prize money in a single season.
The initial signs did not look good for Williams. She was serving often well below 100mph and her movement was decidedly sluggish.
But her biggest problem was how well her opponent was playing.
Li has had a consistently very good season and her victory over Petra Kvitova on Saturday guaranteed she will finish the season at a career-high of number three in the world.
The Chinese player had a spring in her step and her fizzing groundstrokes and clever placement were simply too much for Williams to handle.
After losing the first game, Li reeled off five games in a row and Williams had to save two set points to stop her making that six.
Li double-faulted on her third chance but at the fourth time of asking she clinched the set - the first she had won against Williams for four and a half years.
In their last meeting in the US Open semi-finals, Li managed just three games, but she vowed not to let herself be beaten before she walked on to the court this time.
The opening game of the second set was a long one and a key one, with Williams saving a break point and letting out a yell of 'come on'.
She then broke Li's serve for the first time before holding for 3-0, her serve now back to something like normal speed and her feet slightly less leaden.
Li dug in to pull it back to 3-3 but a loose eighth game cost her and Williams showed composure to save two break points before levelling the match.
The start of the decider was crucial and it was Williams who gained the early advantage, a costly double fault from Li handing the American a break.
And from there it was one-way traffic, Williams completing a run of nine games in a row with a backhand winner down the line before sinking to her knees in elation and exhaustion.
The 32-year-old beat her own record from last year to become the event's oldest winner, and extended her winning streak at the championships to 15 matches.
And lifting the Billie Jean King trophy had a special significance for Williams at the end of the year when the WTA has celebrated its 40th anniversary.
She said: "It was an awesome year of tennis. To end with the Billie Jean King trophy after 40 years and after everything we had the celebration for this year, I don't know if it's written or what, but it's just really exciting.
"I haven't had a lot of time to think about it, but I'm happy. I'm really happy that I was able to finish it off."
Williams' sheer force of will is stronger than any other player on the WTA Tour, and it again helped her turn around a match she looked second favourite to win.
She said: "I felt good this morning, much better than I did yesterday, but then in practice I was like, 'Oh no'. I was a little worried, but I just hung in there and just kept going and going."
Li blamed her own tiredness for the way the match got away from her, saying: "I was feeling maybe too excited at the start of the match. So after one and a half sets I had no more energy left.
"I think it's a tough tournament because I played five days in a row."
Jankovic had made a pointed comment about sportsmanship on Saturday in relation to the fluctuations in Williams' serve and apparent levels of effort, and Li hinted at a similar frustration.
"So this is the reason why she lives in Los Angeles," said Li about the home of Hollywood.
"I don't know what happened about her, but when she started the match she was already feeling like death. But in the final set she can serve 180 or 190kph."