Reigning U.S. Open champion Kim Clijsters put in one of the worst performances of her career and won just one game as she crashed out 6-0, 6-1 to Nadia Petrova in the third round of the Australian Open on Friday.

The Belgian made 17 unforced errors in the first set and barely improved throughout her 51 minutes on court.

"I haven't changed anything in my whole preparations before every match. Everything was the same, same routine, then something like this happens," said Clijsters.

"That's probably the most frustrating thing about it, it's not knowing. That's sports. You know, it can happen."

Compatriot Justine Henin fared much better, however, coming from a set down to beat 27th seed Alisa Kleybanova 3-6, 6-4, 6-2 victory in two hours and 20 minutes, and will now face fellow Belgian Yanina Wickmayer.

Russia's No.2 seed Dinara Safina looked in superb form as she demolished Elena Baltacha 6-1, 6-2 in less than an hour.

Former world No. 1 Jelena Jankovic was upset 6-2, 6-3 by No. 31 Alona Bondarenko, while China's Zheng Jie toppled 2007 Wimbledon finalist Marion Bartoli of France 5-7, 6-3, 6-0.

On the men's side, U.S. Open champion Juan Martin del Potro had a 6-3, 0-6, 6-4, 7-5 win over Florian Mayer.

If he wins in the fourth round, he could meet Andy Roddick in the quarterfinals.

Seventh-seeded Roddick was pushed into four sets before beating Feliciano Lopez 6-7 (4), 6-4, 6-4, 7-6 (3) and will play 2007 runner-up Fernando Gonzalez of Chile, who outclassed Kazakhstan's Evgeny Korolev 6-7 (5), 6-3, 1-6, 6-3, 6-4.

Britain's Andy Murray had a 7-5, 6-1, 6-4 win over Florent Serra of France and will now play American John Isner, who fired 26 aces and hit 73 winners in a 6-1, 4-6, 7-6 (4), 7-6 (5) victory over No. 12 Gael Monfils of France.

Rafael Nadal's bid to defend his title was challenged by German Philipp Kohlschreiber in a closely-fought encounter that lasted well over three and a half hours before the Spaniard progressed 6-4, 6-2, 2-6, 7-5.

"It was a very close match. I'm very happy to get through to the fourth round but it was tough," said Nadal.

"It was long match but I feel okay, although I might be a bit tired tomorrow."