Jelena Jankovic topped second-ranked Victoria Azarenka 6-4, 6-3 in her first match at the WTA Championships on Wednesday.
Fourth-seeded Li Na opened her campaign with a 6-3, 7-6 (5) victory over Sara Errani.
Errani also lost her opening group match on Tuesday to Azarenka and now looks all but certain to be eliminated.
Jankovic, who was ranked No. 1 in 2008 and is back in the top 10 for the first time since 2011, outplayed the erratic Azeranka to snap a four-match losing streak against a fellow former No. 1.
Li trailed 3-1 in the second set but won the next four games - and then couldn't close it out. She twice served for the match - wasting a match point at 5-3 - and both times dropped her serve. Trailing 3-1 in the tiebreaker, Li won the next five points. She converted her third match point when Errani sent a forehand into the net to improve to 6-0 in career head-to-head meetings against her opponent.
The first match is always tight... I still have to find the rhythm on the court
"The first match is always tight," Li said, adding that she had felt rusty after a three-week break since her last tournament.
"I still have to find the rhythm on the court."
The tournament brings together the top eight players in the world. They play two round-robin groups, with the top two finishers in each advancing to the semifinals.
Jankovic, now ranked No. 8 and seeded seventh because third-ranked Maria Sharapova pulled out of the event with a shoulder injury, proved to be the steadier player against Azarenka, the Australian Open champion.
The key game in the first set went to Jankovic after more than 10 minutes when she held serve for a 5-3 lead. Azarenka saved two set points in the next game but then sent a forehand wide on Jankovic's fourth in the 10th game.
Azarenka dropped her serve in the third game to fall behind and then held in the fifth despite serving three double-faults and facing four break points. But she could not trouble Jankovic on her serve and another forehand by Azarenka that sailed long gave Jankovic the match.
After losing the U.S. Open final to Serena Williams, Azarenka lost her opening matches in both Tokyo and Beijing before beating Errani on Tuesday.
Azarenka still has a 5-4 career edge over Jankovic.