But there was none at all for Novak Djokovic, after he threw in the towel midway through the fourth set of his quarter-final with Roddick as temperatures soared to 36 degrees Celsius.

Federer essentially labelled the Serb, the first men's champion to give up a match while defending his title since Pat Rafter at the 1999 US Open, a quitter.

'I'm almost in favour of saying, you know what, if you're not fit enough, just get out of here,' the world No. 2 blasted.

'If Novak were up two sets to love, I don't think he would have retired 0-4 down in the fourth. Thanks to Andy that he retired in the end. Andy pushed him to the limits. Hats off to Andy.'

For the record, Djokovic gave up while trailing 7-6 (7-3), 4-6, 2-6, 1-2.

In a sport that once saw a vomiting Pete Sampras overcome dehydration in a marathon quarter-final on the way to the 1996 US Open title, such excuses are looked at with disdain.

For Djokovic, a pattern is emerging.

In 2006, he retired from his French Open quarter-final against Rafael Nadal after losing the first two sets. A year later, he abandoned his Wimbledon semi-final contest against the Spaniard, blaming a toe blister.

Yesterday, his excuse was that he was cramping and sore, and 'didn't really have time to recover' from his previous match against Marcos Baghdatis which ended at 2.26am on Monday.

But Djokovic insisted: 'I always retire with a reason. I don't see why anybody should criticise me.'

But Federer also took issue with the world No. 4 asking the organisers to let him play his quarter-final during yesterday's evening session, but the organisers put the Swiss on instead.

'At the beginning of the tournament you have a hundred players requesting something. You can't always give the players everything they want,' Federer said. 'I would have been ready to play in the heat. I like playing in the heat.

'Novak had a day off. It's not like he had to play the next day at 11 in the morning. He still had a day and a half off.

'I don't think it's the end of the world.'

Federer showed no ill-effects from his five-set victory over Tomas Berdych in the previous round and had no problems in the cooler conditions, winning the final 13 games against the 20-year-old Argentinian del Potro in a match that lasted only 80 minutes.

The eighth-seeded del Potro, playing in his second Grand Slam quarter-final after losing to Andy Murray at the US Open last year, started with some promise before being broken in the fourth game of the first set.

That seemed to shake his confidence and, although he held serve for two more games after that, he went on to lose the set 3-6 and then went into meltdown.

Federer's mix of speed and spin, on the other hand, were masterful. One service game in the first set summed it up - ace, backhand winner, forehand winner, volley winner, all in less than two minutes. As he served for the second set, a fan shouted: 'You're perfect, Roger!'

Not quite, but very, very good.

'The longer the match went, the more he struggled and the better I got,' said Federer.

In the first match of the day, Vera Zvonareva ran off 11 straight games to beat 2007 Wimbledon finalist Marion Bartoli of France 6-3, 6-0 to reach the semi-finals for the first time in 25 Majors.

The seventh-ranked Zvonareva will meet fellow Russian Dinara Safina, who survived 11 double-faults and 36 unforced errors to beat Australia's Jelena Dokic 6-4, 4-6, 6-4.

Safina apologised to the crowd for beating the local favourite, who advanced through a wild-card play and was ranked No. 187. 'I hope that next time you'll be behind me,' she said.

REUTERS, ASSOCIATED PRESS