French Open champion Myskina and sixth seed Dementieva had been expected to easily join compatriots Maria Sharapova and Svetlana Kuznetsova in the last eight by beating lower-ranked fourth round opponents.

But in the biggest upset of the tournament so far, third seed Myskina was sent crashing out by 19th seeded Frenchwoman Nathalie Dechy, losing a one-sided encounter 6-4, 6-2 on the Rod Laver Arena.

"No problems, just a bad day, a bad game," said Myskina, who committed 45 unforced errors throughout. "Nathalie played pretty smart against me today ... I can't say really that she won the match. I can say that I lost the match."

Dechy was delighted after reaching a grand slam quarter-final for the first time in her career but bristled at Myskina's assertion that the result had more to do with the Russian's own shortcomings.

"I'll just focus on the result," Dechy said.

"I'm in the next round and she's not. I won the match. I played the way I needed to play against her."

Dechy will now play Swiss 12th seed Patty Schnyder, who advanced to the quarter-finals after a dramatic collapse by Dementieva, the losing finalist at the French and US Opens last season.

Dementieva was beaten after leading Schnyder by a set and 4-0 to eventually lose 6-7, 7-6, 6-2.

"I mean, what can I say?" said a disconsolate Dementieva.

"I was 4-0, 30-0. You know, I didn't take my chance."

Elsewhere in the women's draw, top seed Lindsay Davenport got through to a last eight match against Australian Alicia Molik.

Home hope Molik scored the biggest win of her life on Monday as she upset former world number one Venus Williams 7-5 7-6.

Molik, bidding to become the first Australian woman to win the title since Chris O'Neil in 1978, outslugged eighth-seeded Williams to move into the last eight of a grand slam event for the first time.

Molik began the match with an ace and after trading breaks, a backhand over the baseline from Williams handed the 23-year-old a second break and the opening set.

The second set went with serve but Molik played a superb tiebreak, winning it 7-3.

World number one Davenport had too much power for her 13th-seeded opponent, Karolina Sprem of Croatia, and clinched a 6-2, 6-2 victory in an hour.

Davenport, who shelved retirement plans at the end of last season to chase her fourth Grand Slam title this year, said she was encouraged by her display.

"I felt today was a big improvement from my last few matches and I felt I did some things a lot better in terms of groundstrokes and being more aggressive," the experienced American said.

"If my game's improving that's always a good sign."

In the men's draw, third seed Hewitt -- aiming to become the first home winner since Mark Edmondson in 1976 -- was given the fright of his life by Spanish teenager Rafael Nadal before winning a gruelling five-set thriller.

Nadal, the gifted 18-year-old ranked 56th in the world, forced Hewitt to fight back from two sets to one down to clinch an epic 7-5, 3-6, 1-6, 7-6, 6-2 victory in 3hr 53min.

Hewitt's win put him in the quarterfinals for the first time, where he will play the winner of Monday's late fourth-round game between Argentines Guillermo Coria and David Nalbandian.

Hewitt admitted he had been clinging on after falling 2-1 behind.

"I tried to put all the negative thoughts out of my mind, just hang in and wait for the opportunities," Hewitt said, paying tribute to his beaten opponent.

"He had nothing to lose, he's got a great attitude and he's good for the game. He's hungry, he wants to play these matches in front of these big crowds.

"This guy's going to be around for a while."

Earlier, second seed Andy Roddick beat Germany's world number 102 Philipp Kohlschreiber 6-3, 7-6, 6-1 in 1hr 27min and will now take on Russian 26th-seed Nikolay Davydenko in Wednesday's quarterfinals.

Davydenko, who sent British seventh seed Tim Henman tumbling out in the previous round, accounted for another higher-ranked player by beating Argentine 12th seed Guillermo Canas 6-3, 6-4, 6-3.

Roddick, on course for a semifinal in the bottom half of the draw with Hewitt, served up a total of 15 aces in Monday's match, but his error count was a much higher 25.

"I don't think I hit the ball as clean as I did in my first three matches, but I felt like I started hitting the right way in the third set," Roddick said.

"I'm not too concerned, I got through okay."