Serena into semis of WTA Championships; Radwanska finishes 0-3
Williams, the defending champion, won all three matches in her group without dropping a set. She's bidding for her fourth title in the eight-women, season-ending tournament.
Earlier, Li Na outlasted Jelena Jankovic 6-3, 2-6, 6-3 for her second victory and eight-seeded Angelique Kerber upset No. 3 Agnieszka Radwanska 6-2, 6-2 to remain in contention for the semifinals. Radwanska was eliminated.
Kerber (1-1) went 0-3 last year in the tournament. She will play Kvitova on Friday for a spot in the last four.
Williams needed to win a set to qualify and she breezed through the first despite fighting off three break points in the first game. She quickly warmed up to overwhelm the fifth-seeded Kvitova, who won Wimbledon and the WTA title in 2011. Kvitova had to withdraw last year because of injury.
"Something told me to hold serve in that game, and I'm glad I did," Williams said. "Could have definitely changed things. I could still be out there. I really just dug deep and tried to hold.
"When I play top players or a Grand Slam winner, such as Kvitova who has such a dangerous game, you've got to go in there knowing that anything can happen, and I have to be really focused."
Williams broke serve again for a decisive 3-1 lead in the second. She set up a match point with a service winner and won when Kvitova's return sailed into the net. She finished with 11 aces.
"I think her serve is really big, so I knew I wanted to hold. I served well," said Williams, who has never lost to Kvitova in five matches.
The 32-year-old Williams is enjoying the best season in her career, having won 10 titles. She added the French Open and U.S. Open titles to bring her Grand Slam record to 17 championships and has a 76-4 record on the year.
Making her eighth appearance in the event, Williams is looking to become the first player to successfully defend the title since Justine Henin in 2007. Henin was also the last to win 10 titles in a season in 2007.
Williams has clinched the year-end No. 1 ranking for the third time in her career after 2002 and 2009.
Li had a 6-4 career edge over her Serbian friend, but both players struggled with consistency. Li had 45 unforced errors and 21 winners, and Jankovic produced 36 unforced errors and 17 winners.
"We had a great fight," Jankovic said. "In the third set it was just a couple of points. I got broken in that (seventh) game. She lobbed me, and the ball went on the line."
Li followed the lob with a crisp backhand winner to take the game.
"They were the crucial points at that stage and made a difference," Jankovic said.