(Reuters) - 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.

I hope it's not serious, she told reporters. I could feel it today, especially in the third set. I'm going to go get it checked out tomorrow and hopefully it's ok. Let's just hope for the best. I should be completely fine for Australian Open. I started feeling it more and more during the match.

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.

World number eight Radwanska 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, teary-eyed during the final set, floated a forehand long to concede defeat just after midnight.

She faces the prospect of starting the Australian Open as the top seed having relinquished the world number one spot between the draw and play getting underway at Melbourne Park.

Kvitova is guaranteed to knock Wozniackioff the top if she wins the final in Sydney, scheduled for Friday night.

To be honest, I don't think about the ranking, said Wozniacki who has never won a grand slam. The most important thing is to be healthy and be able to play and play well.

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.