Defending champion Elina Svitolina stormed into the semi-finals of the season-ending WTA Finals after a commanding win over Simona Halep on Wednesday, while Canadian sensation Bianca Andreescu retired hurt against Karolina Pliskova.
Andreescu injured her left knee in the third game after attempting to return a wide serve. She received heavy strapping during the subsequent medical timeout.
The 19-year-old gamely fought on but was noticeably hobbled and retired after the first set, which Pliskova won 6-3 in 51 minutes. Winless Andreescu can't qualify for the semi-finals, while Wimbledon champion Halep will play Pliskova to decide the group's second spot.
Andreescu, the youngest player in the draw, said she heard her "knee crack" but was unsure whether she would play the dead rubber against Svitolina on Friday.
"It (the knee) kind of went inwards," she told reporters. "Putting pressure afterwards on it really bothered me. I could barely bend my knee.
"Hopefully I can recover quickly," she added.
Pliskova said she was excited about her upcoming must-win match against Halep.
"Of course, there's going to be some nerves but I think that's normal," she said. "You lose, you go home. There's not going to be anything we're not used to."
Svitolina will finish top of Purple Group after taking advantage of an error-strewn Halep to win 7-5, 6-3 in one hour and 39 minutes at the Shenzhen Bay Sports Centre.
Svitolina is the first player through to the semi-finals, having also beaten world number two Pliskova in straight sets on Monday. Svitolina had the better of long baseline rallies against an uncharacteristically errant Halep, who finished with 38 unforced errors.
"It's about managing attack and defense for me," Svitolina said. "I've been working on this. That's working well this week."
After a slow start, Halep came back into the contest. A powerful forehand winner down the line clinched her break in the fifth game before the players held serve.
Halep misfired sloppily in the 12th game as she handed Svitolina the first set with a slew of unforced errors.
Halep started well in the second set with an early break before falling away against the more consistent Svitolina.
The result broke a deadlock between the pair with Svitolina now holding a 5-4 edge and exacting revenge for a semi-loss to Halep at Wimbledon in July.
The 25-year-old is hoping to end a barren season on a high having failed to win a title since last year's WTA Finals triumph.
Halep said she had to sharpen her game to stay in the tournament.
"I really want to win because I want to qualify from this group," she said.
"Hopefully I can recover because I'm a little bit tired and play better than today," she added.