MELBOURNE, Australia — The much-anticipated potential quarterfinal between the back-from-retirement Belgians dissolved in 52 minutes today at the Australian Open.

That's the time it took for Kim Clijsters to lose, in embarrassing fashion, her third-round match at the season's first Grand Slam tournament.

The 6-0, 6-1 defeat to Nadia Petrova was the worst of Clijsters' career and as one-sided as the score indicated. Clijsters lost the first set in just 18 minutes and won only five points in her first seven games.

Justine Henin, another Belgian who was inspired to return to the tour following a 20-month absence after Clijsters won the U.S. Open, held up her end of the bargain. With some difficulty, she defeated Alisa Kleybanova 3-6, 6-4, 6-2.

"I just didn't give her a chance to get into the match," Petrova said. "I like this kind of Grand Slam feeling."

Clijsters hadn't been on the receiving end of such a lopsided scoreline since losing the French Open final to Henin 6-0, 6-4 in 2003.

"I was completely off. I think tennis-wise, I didn't feel the ball at all," Clijsters said. "On the other hand, she was good, but I made all the mistakes and she really didn't have to do much."

Defending champion Rafael Nadal advanced to the fourth round Friday along with Andy Murray, U.S. Open champion Juan Martin del Potro, Andy Roddick and Fernando Gonzalez.

Nadal, who beat Roger Federer in last year's final, held off Philipp Kohlschreiber of Germany 6-4, 6-2, 2-6, 7-5 in a night match.

In the fourth round, Nadal will play Ivo Karlovic, who beat Ivan Ljubicic 6-3, 3-6, 6-3, 7-6 (7). The 6-foot-10 Karlovic holds the tour record for the most aces in a match with 78.

"He's a really difficult opponent because the match is decided in a few points," Nadal said. "So the thing is to concentrate with my serve all the time and try to convert the small opportunities that I can have on the return."

The fifth-seeded Murray, bidding to win his first Grand Slam singles title, beat Florent Serra of France 7-5, 6-1, 6-4 and is one victory away from a potential quarterfinal with Nadal.

First, Murray will have to beat American John Isner, who won at Auckland last week for his first ATP title. The 6-foot-9 Isner continued his strong play by defeating 12th-seeded Gael Monfils of France 6-1, 4-6, 7-6 (4), 7-6 (5).

"I was just kind of riding a wave of momentum, and I still am," Isner said. "Hopefully, I can keep it going."

No. 4 Del Potro beat Florian Mayer of Germany 6-3, 0-6, 6-4, 7-5 and No. 7 Roddick advanced 6-7 (4), 6-4, 6-4, 7-6 (3) over Feliciano Lopez of Spain.

Next up for del Potro is Marin Cilic, who stopped Stanislas Wawrinka 4-6, 6-4, 6-3, 6-3.

Gonzalez beat Evgeny Korolev 6-7 (5), 6-3, 1-6, 6-3, 6-4. Roddick and Gonzalez will play each other in the fourth round.