ISTANBUL - Caroline Wozniacki, who celebrated becoming year-end world No 1 for the second successive year on Wednesday, marked the occasion by crashing out of the WTA Championships on Thursday.

Although the 21-year-old Dane has topped the rankings for 64 weeks, she was so outplayed by Petra Kvitova, the Wimbledon champion from the Czech Republic, that she looked anything but the best player in the world.

Indeed there were moments when Wozniacki was so out of sorts during a 6-4, 6-2 defeat that the crowd began to applaud her out of sympathy.

Twice she called her father-coach, Piotr, on to court and once she asked for the trainer, who appeared to take her blood pressure, and the setback left Wozniacki with only one victory in her three group matches.

Kvitova's win ensured she qualifies for the semifinals after two matches, and Vera Zvonareva, the Russian who beat Wozniacki on Wednesday, and Agnieszka Radwanska of Poland, would finish above the top ranked player.

Radwanska defeated Zvonareva 1-6, 6-2, 7-5 to set-up a tense Friday finale to the group stage.

If Radwanska wins just a set against Kvitova on Friday, she will qualify for the semifinals; if Radwanska loses to Kvitova in straight sets on Friday, Zvonareva will go through.

Wozniacki's dominance of the rankings was described as "a huge personal achievement" by Stacey Allaster, the chief executive of the WTA, with her name being linked to a group of only seven women - including Martina Navratilova and Steffi Graf - who have finished consecutive seasons as No 1.

Wozniacki called it "a dream come true", but it would arguably have been as big a boost for her to win the WTA Championships, a bigger title then any she has yet won.

It was doing this which notably inspired Amelie Mauresmo, the now retired French favorite, to win two Grand Slam titles within a few months of capturing the WTA Championships title in Los Angeles in 2005.

Rather like Mauresmo then, Wozniacki has conspicuously failed to win a Grand Slam.

Here she has looked like a physically gifted player, with excellent movement, tactical sense and consistency, but without a cutting edge or a weapon to drag her out of trouble.

It may precisely be this hard-working style which, after almost 80 matches in a year, has contributed to the worn-out batteries which seemed evident in this flat performance.

"I tried my best, but my body didn't want to do the things I asked it today," 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."

Kvitova was indeed destructive with her flat-hitting, even on a slowish court, and her angular left-handed serve was very deceptive. In this vein she may be hard to stop.

However, it has been Victoria Azarenka who has been looking the strongest player in the field at the moment.

Her 6-2, 6-2 win over Li Na, the French Open champion from China, took little more than an hour and a half, guarantees a semifinal place, even if she were to lose on Friday to Marion Bartoli of France, who replaces of Sharapova.

Agence France-Presse