Caroline Wozniacki faced a battle to be fit for the Australian Open after being reduced to tears during a dramatic defeat by Agnieszka Radwanska at the Sydney International on Wednesday.
Wozniacki lost 3-6 7-5 6-2 in a late-night quarter-final thriller, which could signal the end of her reign as world number one with Czech Republic's Petra Kvitova in position to take top spot within 48 hours.
The Dane's left wrist was strapped when trailing 1-2 in the deciding set, just five days before the start of the year's first grand slam.
Top seed Wozniacki served for the match at 5-4 in the second set before Poland's Radwanska hit a purple patch to run away with victory in their quarter final.
World number eight Radwanksa will play Belarus's world number three Victoria Azarenka in the semi-finals on Thursday. World number two Kvitova lines up against China's world number five Li Na.
Wozniacki floated a forehand long to concede defeat just after midnight. She was teary-eyed during the final set after receiving treatment to her wrist.
"I'm really happy, thank you to the people who stayed to watch us so late," Radwanska told reporters. "Caroline is a great person and a great player but I'm very happy to be through to the semi-finals."
Kvitova is guaranteed to end Wozniacki's stay at number one if she wins the final in Sydney, scheduled for Friday night.
The top seeds were expected to slug it out in the championship match but Wozniacki is now powerless to influence the result.
Kvitova was highly impressive in her 6-0 6-4 thumping of Slovakia's world number 21 Daniel Hantuchova on Ken Rosewall Arena.
"Really I played very well," Kvitova said. "On this high level, how I played the first set, it's not easy to play like that all match.. I actually thought before tonight I hated playing in the wind but I played not bad.
"You have to fight with the wind. Nobody is asking if it's okay for you or no. I'm still on track (to be number one) but it's still two matches to go and the next one is really tough against Li Na. She's a tough opponent, of course. She's in the top five."
Kvitova had struggled with her breathing in her three-set win over Alexandra Dulgheru in the second round, a match played in the heat of the day. Her asthma was much less a factor in the night match against the overpowered Hantuchova.
"I have to say it was not really bad tonight," she said. "I have more problem in the USA than here. This is not really humid here."