World number one Caroline Wozniacki crashed out of the WTA Championships in Turkey after losing to Petra Kvitova.
Denmark's Wozniacki, 21, who has played 80 matches on tour this year, needed her blood pressure checked after the first set and ultimately lost 6-4 6-2.
Wozniacki's second loss in the season-ending event came as the Czech Kvitova sealed her last-four place.
Victoria Azarenka of Belarus beat China's Li Na 6-2 6-2 to book her semi-final spot with a match to spare.
Also in Azarenka's White group in the elite eight-player event, French Open champion Li will play US Open winner Samantha Stosur on Friday with a semi-final berth at stake.
In Kvitova's Red group, Agnieszka Radwanska of Poland beat Russian Vera Zvonareva 1-6 6-2 7-5 to set-up a tense finale on Friday.
Radwanska needs to win just a set against Kvitova to qualify for the knock-out stages, but if the Pole loses in straight sets Zvonareva will go through.
Wozniacki, who will end the year on top of the rankings for a second consecutive season, found the effects of two long three-set matches in two days too much to cope with.
She only won 54% of points on her first serve and failed to create a single break point against Kvitova.
"I tried my best, but my body didn't want to do the things I asked it," Wozniacki said. "What I told my brain didn't go to my body.
"A lot of things have been happening this week. It's just unfortunate that my body has been feeling tired. To get sick now is not the best time if you want to beat the top players.
"That's not to say Petra would not have won the match anyway. She has been playing very well and she deserves to have won her two matches."
Azarenka, who lost to Li in the fourth round of the Australian Open and in the quarter-finals in Paris, is guaranteed a semi-final place, even if she loses to Maria Sharapova's replacement Marion Bartoli of France on Friday.
"Li Na has such a good return and likes to dominate from the beginning, so I tried to serve well and take that away from her," said Azarenka. "The crowd here in Istanbul just makes me want to play well and show them some good tennis.
In the late match, Zvonareva squandered her chance to secure a semi-final place after losing three match points to a resurgent Radwanska.
Zvonareva served for the match at 5-3 but Radwanska rallied to seal her fourth successive game and clinch victory in two hours 26 minutes.
"Every point, every shot I just kept fighting to the end," she said. "I'm just so happy that I could come back and win this match."