Williams sank to her knees after firing a backhand winner on her second match point. She became the eighth woman to win 11 or more titles a year and the first since Martina Hingis had 12 in 1997.
This was Williams’ fourth WTA Championship. She also won the elite tournament, which features the eight top-ranked players, in 2001, 2009 and 2012. She is the first to successfully defend the WTA title since Justine Henin in 2007.
Williams has won 78 of 82 matches this year. She will finish the year as the top-ranked player for the third time, the others in 2002 and 2009. Li, playing in her first WTA Championship final, will finish the year at a career-high No. 3.
The final featured two players over 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 against a sluggish Williams, who struggled a day earlier to beat Jelena Jankovic and complained later of exhaustion after a long season.
The Chinese, passing Williams almost at will, 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. The American fended off two break points before finally prevailing with a service winner.
Li ripped an ace, her only one of the match, to make it 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.
It was another double-fault by Li that gave Williams a break of serve at the start of the third, and Williams was on her way.
SWISS INDOORS: In Basel, Switzerland, Juan Martin del Potro denied Roger Federer his hometown title for the second consecutive year, winning the final 7-6 (3), 2-6, 6-4 on Sunday.
The top-seeded Argentine clinched the title when Federer could not return a tough forehand.
Federer had hoped the raucous support at Basel, where he is a five-time champion, could help revive his difficult season. He’s won just one title in the past 14 months.
Still, the sixth-ranked Swiss star edged closer to securing his place in the eight-man lineup at the ATP World Tour Finals in London starting Nov. 4.
Del Potro earned his fourth title this season, all on his favored hard-court surface. He also won at Rotterdam in February, Washington in August and Tokyo this month.
With this loss, Federer has only one victory against a top-10 opponent in 2013. That was back in January, against then-No. 8 Jo-Wilfried Tsonga in the Australian Open quarterfinals.
Though the 32-year-old Federer still holds a 13-5 record against del Potro, he has lost the past three including the 2012 Basel final.
VALENCIA OPEN: In Valencia, Spain, Mikhail Youzhny beat top-seeded David Ferrer 6-3, 7-5, to win the title.
The 31-year-old Russian dominated the No. 3-ranked Ferrer throughout, breaking the defending champion and home town favorite four times on the indoor hard court.
Youzhny took a one-set lead by breaking Ferrer twice early on, picking the Spaniard apart with well-placed ground strokes.
Ferrer appeared to have recovered with a break in the second set. But Youzhny recovered his service game to level the score and broke Ferrer a final time to go up 6-5 before serving out the decisive game.
Youzhny claimed his 10th career title when Ferrer netted his final return.
Youzhny had finished runner-up here to Andy Murray in 2009.