Li Na booked her place in the semi-finals of the WTA Championships with a 6-2 6-1 win against an ailing Victoria Azarenka in her final match of the round-robin.
The Chinese player's third straight victory also confirmed Jelena Jankovic as the other player from the White Group to make it through to the last four.
Jankovic progressed despite losing the final match of the day - by this stage a dead rubber for both players - to the previously winless Sara Errani 6-4 6-4.
Jankovic, Errani and Azarenka all ended up with one win but the Serb advanced courtesy of her superior sets record.
The final place in the semi-finals was taken by Petra Kvitova, who beat Angelique Kerber 6-7 (3/7) 6-2 6-3 in a straight fight for second place in the Red Group behind World No.1 Serena Williams.
World No.2 Azarenka hurt her back while serving in the sixth game against Li, needing a medical timeout and treatment during several changeovers.
She only managed to win one game after the injury as she crashed to her third defeat to complete a miserable tournament.
Azarenka, who was wincing in pain and in tears at one point, said: "I just wanted to try to do my best for the fans who came and watched our match, for respect for my opponent. It was just about trying to do the most you can out there."
Li said: "Today the match is not about tennis, about fitness ... because I thought maybe after the first set she will give up or retire, but she still try to continue to play.
"I think it's tough because I have to focus, hope all time, because you never know what happen on the court."
Williams was the first player into the last four on Thursday when she beat Kvitova to maintain her 100 percent record.
Williams will meet Jankovic in the semi-finals on Saturday, with Li taking on Kvitova.