Top-ranked Victoria Azarenka saved two match points before rallying past Angelique Kerber of Germany 6-7 (11), 7-6 (2), 6-4 on Wednesday in her first match at the WTA Championships.
Azarenka needs just one more round-robin win to secure the No. 1 ranking for the year.
In another Red Group encounter, Serena Williams proved that even when her serve is malfunctioning, she can still beat a top player.
Williams beat 2011 French Open champion Li Na of China 7-6 (1), 6-3 for her second win in round-robin matches to close in on the semifinals at the tournament for the top eight players in the world. Maria Sharapova and Agnieszka Radwanska were scheduled to face off in the day's final match.
Williams had six double-faults and won only 51 percent of points on her first serve. But Li also struggled on serve -- there were eight breaks in the first set and three more in the second.
''My serve just didn't go in. Every time I got my first serve in, I was surprised,'' Williams said. ''But knowing I was able to still, for me, zero serve compared to how I normally serve -- I think it's something good I can take out of the match.''
She has won 28 of her past 29 matches and captured Wimbledon, US Open and Olympic titles this year. The highest ranking she can achieve at the end of the year is No. 2 because of injuries and missed tournaments.
At one point, Williams got so frustrated at dropping serve that she uncharacteristically smashed her racket.
''No, I don't break rackets in matches. Although I didn't hit it hard,'' she said. ''Maybe I don't know my own strength.''
The first set was full of missed opportunities. The eighth-ranked Li took a 4-1 lead before Williams won the next four games. Serving for the set, the American again dropped serve. Li was unable to hold, though, and Williams had another chance to serve out the set -- only to lose it at love.
Williams took control of the match in the second set by saving a breakpoint -- in a game that lasted nine minutes -- to hold and lead 4-1.
The American wasted two match points to break Li, but closed it out on her own serve when the Chinese player sent a forehand long.
Both Azarenka and Kerber smashed rackets in the second, highly entertaining match.
Kerber is making her debut in the event after winning two titles and climbing to No. 5 this year after finishing 2011 at No. 32. She was down a break in the second set, but led 5-4 after prevailing in a game that went to nine deuces and lasted 13 1/2 minutes.
Azarenka then saved two match points before capturing the second tiebreaker. The Belarusian closed it out on Kerber's serve, hitting a forehand winner on her first match point.
Earlier, defending champion Petra Kvitova withdrew from the tournament because of a viral illness and was replaced by Samantha Stosur.
The event splits players into two groups, with the top two in each advancing.