Wimbledon champion Roger Federer has been bounced by his Argentine bogeyman bringing his U.S. Open run to a halt but Kim Clijsters was on fine form as rain clouds finally cleared from Flushing Meadows.

Federer, seeded second here on the back of his grasscourt Grand Slam triumph, fell once again to Argentine David Nalbandian, this time 3-6, 7-6, 6-4, 6-3 in the fourth round.

The stocky 21-year-old from Cordoba has now beaten Federer all five times they have met on the Tour -- three times this year and twice in 2002.

"I guess I am struggling against him," Federer smiled wryly.

"I don't know what to comment on this.

"I am trying to figure out how to beat him. He likes my game, that's all I can say about it."

Thirteenth seed Nalbandian, himself a finalist at Wimbledon last year, was also at a loss to explain his hold over the stylish Swiss player.

"I like playing against him, what can I say?" he laughed.

"I like his game... I don't know exactly why."

Clijsters reached her first U.S. Open semi-final with a comprehensive 6-1, 6-4 victory over France's Amelie Mauresmo.

The Belgian top seed will face former champion and third seed Lindsay Davenport for a place in Saturday's final after the American ousted Argentine Paola Suarez 6-4, 6-0.

"It's definitely a great feeling," Clijsters grinned.

Her fellow Belgian and second seed Justine Henin-Hardenne was also feeling great having booked her spot in the last four with a 6-2, 6-3 win over Anastasia Myskina.

SECOND-HAND OPPONENT

Myskina had earlier in the day completed her fourth round victory against Mary Pierce.

The Russian seventh seed won that contest, which spanned four days, 7-6, 6-1 but was no match for Henin-Hardenne under the floodlights.

French Open champion Henin-Hardenne will face Jennifer Capriati in the last four after the American also beat a "second-hand" opponent.

Italy's Francesca Schiavone had also played earlier on Thursday, completing a 6-7, 7-5, 6-2 win over seed Ai Sugiyama.

Like Myskina, Schiavone could not duplicate that form later in the day and sixth seeded Capriati won 6-1, 6-3 in straightforward fashion.

"I played tough... I had a lot of energy out there," Capriati said.

"Even though I am here in the US Open, I don't feel I have any of the pressure," she added, looking ahead to the semis.

"I've just got to go out there and give it all I've got."

Australia's Lleyton Hewitt underlined his revival in form, beating Thai Paradorn Srichaphan 4-6, 6-2, 6-4, 6-2 in a fourth round clash.

The 2001 U.S. Open champion has endured a poor grand slam year and this result gave him his best run at a major in 2003.

"It took me a few games to get into it but then I really started working it well," Hewitt said.

He next meets third seed Juan Carlos Ferrero after the Spaniard beat American Todd Martin 6-2, 6-4, 3-6, 5-7, 6-3.

Federer, who also lost to Nalbandian in the final of the junior event here in 1998, slowly lost his way on a dank centre court, bowing out in two hours 50 minutes.

Nalbandian allowed his opponent to punch himself out before thrashing the heavy tennis balls all over the court to reach his first U.S. Open quarter-final.

"I'm ready... it's time to start talking of Nalbandian as a favourite," he said.

CROWD FAVOURITE

In the last eight he will face crowd favourite Younes El Aynaoui.

The Moroccan, who is developing a cult following in New York, reached his second straight U.S. Open quarter-final by beating former world number one Carlos Moya 7-6, 7-6, 4-6, 6-4.

The 31-year-old cradled his racket and rocked it like a baby after striking a winning backhand on match point -- a reference to the birth of his third son earlier this month.

"There's not many players have a crowd as enthusiastic as this," El Aynaoui said. "I apologised to Carlos afterwards if they disturbed him but he told me it was fine.

"They were definitely very helpful."

Guillermo Coria fought off Jonas Bjorkman to win his fourth round match 6-2, 6-3, 4-6, 6-2 and earn a quarter-final against top seed Andre Agassi.

"It is going to be tough because I am playing against my idol in his favourite tournament, at his home," Coria told reporters.

America's other hope for silverware, Andy Roddick, will meet Sjeng Schalken, a 6-1, 4-6, 6-3, 6-4 winner over German Rainer Schuettler.

Agassi and Roddick advanced on Tuesday and Wednesday respectively.

Grim conditions had once again greeted players and tennis fans when they arrived at a water-logged National Tennis Centre.

But the rains finally cleared and gave organisers a glimmer of hope of finishing the tournament this weekend on schedule.

Before Federer and Nalbandian took to court, only five singles matches had been completed since Monday because of severe rain delays, forcing officials to seriously consider holding the men's final early next week.

But the action was thick and fast on Thursday evening. Matches were sprawled on to far-flung courts as organisers caught up on the schedule to put a Sunday finish back on track.