Lleyton Hewitt's Australian Open dreams vaporised on a stiflingly-hot centre court on Monday when the world number one ran out of answers and ammunition against a 31-year-old Moroccan.

Younes El Aynaoui, better known for his ready smile than a ruthless nature, pummelled the top seed into submission for a 6-7, 7-6, 7-6, 6-4 fourth round win.

"Yeah, disappointing ... there's no other way of looking at it," the 21-year-old said afterwards.

"It might make me a better player," he mused, still red in the face and searching for a positive.

El Aynaoui, by contrast, could not have looked more content, into the quarter-finals of a grand slam for only the third time in his 13-year career.

"I think this might be a surprise to most people, I mean how many people know me around the world? Not very many," he grinned.

"But people who know me would know I had a chance. I kept on believing I could do this."

The Moroccan now faces American Andy Roddick, who needed a mammoth effort to prevent him from becoming another casualty on a day so hot play was suspended on outside courts.

The ninth seed clawed his way back from a two-set deficit to overcome Russia's Mikhail Youzhny as he and Serena Williams led a U.S. charge on the Australian event.

SMOKE-FILLED

Williams walloped Greek Eleni Daniilidou 6-4, 6-1 to reach the quarter-finals. Roddick prevailed 6-7, 3-6, 7-5, 6-3, 6-2.

Both toiled in a smoke-filled atmosphere as bushfires 200 kilometres to the north left a noxious haze over the city.

But neither complained of the conditions afterwards even though at one stage during their matches tournament officials invoked their Extreme Heat Policy to protect players' health.

While the Americans toiled away on the showcourts, tournament referee Peter Bellenger suspended the start of new matches on outside courts for two hours.

The policy kicks in once the mercury hits 35 degrees Celsius and other conditions -- including levels of wind speed and humidity -- are met.

"It's definitely very hot out here," Williams said. "But it could be worse. I live in Florida where when you walk out you lose 20 pounds because of the humidity and heat."

She added that the smoke-filled air had not caused her any difficulty breathing.

"I looked out my window this morning and didn't realise it was smoke," she said. "I thought it was foggy. I think it was a bit strange, but playing-wise I just blocked it. It wasn't a problem for me."

Williams, looking to win the only grand slam title not in her possession, faces fellow American Meghann Shaughnessy in the last eight after Shaughnessy beat Russian Elena Bovina in three tough sets.

RUNAWAY LEADER

Hewitt had won his second and third round matches for the loss of just seven games.

Growing in confidence and getting into a groove, nothing could have prepared him for the clattering he was going to get on centre court.

El Aynaoui primed his guns and came out firing. That he kept it up for three-and-a-half hours was staggering.

Hewitt finished last year ranked number one in the world for a second successive year -- it was a position he held for the entire 12 months, a runaway leader.

But the Barcelona-based Moroccan, ranked 22nd in the world, employed the most simple of game plans to crack the top seed: hit hard, then hit harder.

He sent 75 clean winners thundering past Hewitt, the best retriever in the game, and pummelled 33 aces down.

Hewitt's hustling was redundant. El Aynaoui refused to be drawn into long baseline battles, preferring to pull the trigger on his flailing forehand whenever possible.

More often than not the ball flew past Hewitt's outstretched racket and, as the Australian howled in fury, frustration took over.

El Aynaoui won two of the first three tiebreak sets and finally got the only break of the match for 4-3 when Hewitt nervously double-faulted.

Serving for the match, nerves gripped.

"Yeah you could say I was nervous," he laughed afterwards. "You start thinking of things you shouldn't be thinking of."

But an acrobatic jump smash earned him a first match point and he converted it first time as Hewitt pushed a backhand wide.

"This is just fantastic," he said coming off court, flashing his bright, broad smile. "A great win."