MELBOURNE: Defending champion Kim Clijsters ousted top seed Caroline Wozniacki and ended her controversial reign as world No. 1 with a straight-sets win in the Australian Open quarterfinals Tuesday.

Clijsters held off a determined fight-back from the 21-year-old Dane to win 6-3, 7-6 (7/4) in 1 hour 45 minutes and set up a semifinal against third seed Victoria Azarenka, who beat Poland’s Agnieszka Radwanska.

By failing to reach the last four, Wozniacki is guaranteed to lose her No. 1 ranking to Azarenka, Petra Kvitova or Maria Sharapova when the new standings are released next Monday.

Wozniacki has occupied the top spot since Oct. 11, 2010, apart from one week last February, but she has faced constant questions over her right to be considered the best women’s player, because she has never won a major title.

But the defiant Dane, who lost her only Grand Slam final to Clijsters at the 2009 U.S. Open, backed herself to bounce back by the end of the year.

“You know, to be honest, I don’t really think about it,” a defiant Wozniacki said. “I have been there for a long time already. I finished No. 1 two years in a row ... We are just in January. In the end of the year you see who has played the best, most consistently all year round. I will get it back eventually, so I’m not worried.”

Wozniacki played her usual counter-punching game Tuesday with plenty of retrieving from the baseline, but she didn’t have a weapon to damage Clijsters, who will retire this year and plans to end her career with a flourish.

The four-time Grand Slam champion showed no ill-effects from the ankle injury she suffered in her fourth round match against China’s Li Na.

Clijsters broke Wozniacki four times in the first set as she dominated her younger rival on both her forehand and backhand sides. And although Wozniacki broke twice herself, she was always playing catchup and never looked like taking the set.

The second set followed a similar pattern, but Clijsters faltered while serving for the match at 5-3.

The tie-break went with serve until 4-4, when Clijsters stepped up to win the next three points and seal victory with a simple put-away into the open court.

“It didn’t feel like being up a set and 5-2,” the Belgian said. “I really had to work at it. Caroline changed her tactics [in the second set] and became a bit more aggressive.”

Earlier, Azarenka lost the first set against the eighth-seeded Radwanska, but she came storming back to win 6-7 (0/7), 6-0, 6-2 in searing heat in the Rod Laver Arena.

“We can’t play each other without it going to three sets, we have to push each other to the limit,” said Azarenka, who has now played six three-setters with Radwanska in their 10 meetings.

In a bewildering first set, neither player appeared comfortable on serve and breaks came thick and fast despite both landing a high percentage of first serves, 73 percent for Azarenka and 76 percent for Radwanska.

But after sharing eight service breaks Radwanska was able to lift when it counted and she raced through the tie-break without dropping a point.

Former champion Maria Sharapova and Ekaterina Makarova will contest an all-Russian quarterfinal Wednesday, while Wimbledon title-holder Petra Kvitova plays Sara Errani of Italy.