NEW YORK: Rafael Nadal flexed his muscles at the US Open Thursday with a show of power rarely seen in the modern game.

Showing no mercy against his unseeded opponent, the Spaniard stormed into the quarter-finals with a ruthless display then led a player revolt that pressured organisers into rearranging the schedule after the two previous days were washed out by rain.

The defending champion was quick to express his displeasure after weather delays left him facing the daunting prospect of having to win four matches in as many days to retain his title.

“That's not fair, but that's what it is,” Nadal growled after thrashing Luxembourg's Gilles Muller 7-6 6-1 6-2 in a little over two hours.

“If you don't have rest, you have a big chance not (to) be fit enough to play well (in) the next match.”

Andy Roddick and Andy Murray were also unhappy at the schedule, which they said favoured players in the top half of the draw, who would only have to play three times in four days.

Within hours, the US Tennis Association (USTA) announced they had bowed to the player's demands.

The women's final, originally scheduled for Saturday, was put back a day, while the men's championship decider was shifted from Sunday, the 10th anniversary of the Sept. 11 attacks, to Monday, marking the fourth year in a row the last major of the year has gone into overtime.

“We revised the schedule for the remainder of the 2011 US Open in an effort to be fair to the players and our ticketholders,” tournament director Jim Curley said in a statement.

But not all the players were impressed with the decision, including world number one and title favorite, Serbian Novak Djokovic.

“I'm not really happy about that, to be honest,” he said.

The scheduling changes were in stark contrast to events on court, where almost everything went according to the script.

Djokovic became the first man to reach the semi-finals when his Davis Cup team mate Janko Tipsarevic retired while trailing 7-6 6-7 6-0 3-0 because of a thigh injury.

It was the first time in the tournament that Djokovic had lost a set but the result was never in doubt and he will play Roger Federer in Saturday's semis.

RODDICK IMPRESSES

Federer advanced to his eighth straight US Open semi-final with an impressive 6-4 6-3 6-3 win over Frenchman Jo-Wilfied Tsonga to stay on course for a possible sixth title.

“It's always a great match-up between the two of us. We've played over 20 times, I think, so there is no real secrets out there,” Federer said of his Djokovic encounter.

“We have had particularly good matches here at the US Open and I have the feeling it's going to be something similar again.”

Serena Williams overcame a wobbly start with her serve, in which she was broken in each of her first three games, to defeat Anastasia Pavlyuchenkova of Russia 7-5 6-1.

In the semi-finals, the American faces world number one Caroline Wozniacki after the Dane beat Germany's Andrea Petkovic 6-1 7-6.

Australia's Sam Stosur demolished world number two Vera Zvonareva 6-3 6-3, stretching her winning streak over the Russian to eight matches, and looms as the dark horse after some impressive displays.

She will next play unseeded German Angelique Kerber, who upset Italy's Flavia Pennetta 6-4 4-6 6-3 in their quarter-final.

Murray produced his best display of the tournament in beating American wildcard Donald Young 6-2 6-3 6-3 but Roddick and John Isner kept the Stars and Stripes fluttering in the men's draw by beating higher-ranked European opponents.

Roddick showed glimpses of the form that saw him win at Flushing Meadows in 2003 as he ousted Spanish fifth seed David Ferrer 6-3 6-4 3-6 6-3 to set up a quarter-final clash with Nadal.

The towering Isner beat French 12th seed Gilles Simon 7-6 3-6 7-6 7-6 in a match unsurprisingly dominated by his serve and advances to face Murray.

Roddick's match was supposed to be played on the Louis Armstrong Stadium but was moved to an outside court with less than 600 spectator seats after farcical scenes.

The big-serving American joined forces with Nadal and Murray to berate the tournament referee Wednesday when they were asked to play in light rain and let his feelings known again when the Louis Armstrong Stadium court started leaking water and staff tried in vain to dry the playing surface.

“Find us a court,” Roddick snapped.

“At a certain point that has to take precedence over what people can watch.”