The American won her fourth WTA Championship, having captured the elite tournament for the top eight players also in her debut in 2001 and in 2009 and 2012.
Williams has won 78 of 82 matches this year and will finish the year as the top-ranked player for the third time in her career after 2002 and 2009. Williams' titles this year included the French and U.S. Opens for a total of 17 Grand Slam crowns. Her winning percentage of .951 is the best on the tour since 1990. Martina Navratilova had an 86-1 record and a .989 percentage in 1983.
Still, Williams said she would have preferred to have won at least one more Grand Slam title.
"I live to win slams. I mean, obviously I'm so excited to be the WTA champion," Williams said. "I can't say it's the best (season). I can't say it's not the best. I don't know. I really don't know."
Li was in her first WTA Championships final and will finish the year at a career-high No. 3.
The final featured two players above the age of 30 for the first time in the championships' 43-year history. Williams is 32 and Li is 31 but both are playing some of their best tennis.
Li got off to a better start over a sluggish-looking Williams, who struggled a day earlier to beat Jelena Jankovic and complained later of being exhausted after a long season.
"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," Williams said.
The Chinese was quick to break serve for a 2-1 lead and rolled through the first set, which ended with Williams netting a forehand and hitting a backhand passing shot wide.
Williams won a key game at the start of the second set that went to nine deuces and lasted nearly 12 minutes as the American fended off two break points before finally prevailing with a service winner.
"I guess looking back it was definitely a very important game for me to win," Williams said. "She had been on a real roll. I guess I was determined to win that game at any cost."
Li blasted an ace, her only one of the match, to level at 3-3. But she produced a double-fault and a forehand error to give Williams a chance to serve out the set. Williams needed three set points to close it out.
"After a set and a half, I was feeling no more energy left. It's a tough tournament, because I played five days in a row," Li said. "Every opponent (is in the) top eight, so you never have an easy match in this tournament."
It was another double-fault by Li that gave Williams a break of serve at the start of the third and the American never looked back.
Williams did have to save five break points, the last one with her sixth ace, before wrapping up the title.
Li, the 2011 French Open champion, fell to 1-10 against Williams. She lost a set at 6-0 for the second straight time they played. She won only three games in her loss to Williams in the semifinals of the U.S. Open.
"I have a little bit bad taste, because it's at love, but I still feel more positive things," Li said.
After three years in Istanbul, the tournament moves to Singapore for the next five years.
- AP
