Williams collected her 11th title of the year and became the first player to defend the title since Justine Henin in 2007.
Serena Williams, who claims to be exhausted near the end of a long season, reacted with fretful worrying, and sometimes distracting behaviour while struggling to a 6-4, 2-6, 6-4 two-hour win over Jelena Jankovic.
The world No.1 overcame physical difficulties, emotional distress as well as a stubborn opponent in Jelena Jankovic before reaching the final of the WTA Championships with a 6-4, 2-6, 6-4 win.
The fourth-seeded Li Na will play 2011 champion Petra Kvitova, who came from behind to beat Angelique Kerber 6-7 (3), 6-2, 6-3.
Serena Williams was relentlessly focussed against 2011 winner Petra Kvitova, hitting 27 winners and 11 immaculately delivered aces.
Li Na is already the first Chinese singles player to have won a Grand Slam title and to have reached the world's top five.
The world number two from Belarussia admits to having become over-tired near the end of the season, and once the intelligent and strongly-focussed Jankovic had tenaciously clung to her break to take the first set she began to make steady progress.
The world number one from the United States took little more than an hour and allowed only four games to Angelique Kerber, who beat her in Cincinnati last year but who was now outplayed almost from the moment she dropped serve in her opening service game.
The 26-year-old Russian -- presently number three in the world -- had had to withdraw from the US Open on the eve of the tournament because of the injury and owing to a previous hip problem has played just one match since a second round exit at Wimbledon in June.
The world number five booked her place in next month's WTA Championships in Istanbul featuring the top eight women's players.
Del Potro stretched his Washington win streak to 14 matches by adding to the titles he won in the US capital in 2008 and 2009.
The unseeded Indo-German pair, which had shocked top seeds, lost 4-6 6-7(1) to the Filipino-Briton combo in the second round of the USD 1,295,790 hard court tournament.
They next face Treat Huey of Philippines and Briton Dominic Inglot in the quarter-finals.
Somdev, who created a record by retaining the NCAA title beating Isner 7-6 (7), 4-6, 7-6 (2) in 2007, was overpowered in their first meeting after that collegiate final here Thursday.
Somdev got the better of American Jesse Witten with a 7-5 6-0 scoreline. Sanam Singh had disappointment in store as he lost 0-6 5-7 to Australian 12th seed Samuel Groth.
It is also a major addition to the portfolio of sports events in Singapore, which bills itself as an Asian events capital, alongside its Formula One grand prix and international golf tournaments.
Stephens said Williams has not spoken to her since the 20-year-old beat 15-time major winner in the first Grand Slam of the year.
Vinci's 6-3, 6-7, 6-3 win over Czech Republic's Lucie Safarova handed Italy a 3-1 lead ahead of the dead doubles rubber to be played on Monday.