Djokovic pulled out of his quarter-final with Andy Roddick looking pale and sickly after two-and-a-half hours of toil in hot and muggy conditions with the mercury rising to 35 C (95 F).

The dramatic withdrawal comes a day after Belarus's Victoria Azarenka pulled out in tears after falling ill during her match with Serena Williams.

Azarenka, who was a set up, was staggering around crying before she gave up playing and was helped from the court.

"Conditions were extreme today. It did affect me more than him, as you could see," said Djokovic, blaming cramp for the withdrawal.

"But that was the situation. I just have to cope with it. It was unfortunate for me. I did request to a play night match, but it didn't came up good for me."

The hot weather is expected to worsen with temperatures forecast to rocket to 42 C (111 F) by Thursday.

Melbourne's Herald Sun newspaper said the city would sizzle at more than 40 C for four consecutive days, the longest spell of extreme heat since 1908.

"It is the worst heatwave most will have lived through," senior forecaster Terry Ryan said in a front-page story headlined ‘Heat Wave Hell’.

France's Marion Bartoli was another player to hit out at the conditions after starting her quarter-final with Vera Zvonareva at 12:30 pm.

"It was quite, quite hot. I have to say it was definitely tough conditions," Bartoli said.

"I don't think it's really fair to have one quarter-final played at 1:00 pm, right in the middle of the heat, and one playing at 7:30 pm. But I guess that's the way it is."

After criticism by top players and WTA boss Larry Scott in 2007, match referees can now halt matches when the so-called 'wet bulb' measure combining heat and humidity reaches a certain level.

However, no matches have been stopped despite several scorching days in the middle of the Australian summer.

Last week Marcos Baghdatis said he felt physically sick because of the heat, and women's number one Jelena Jankovic said her feet felt like they were on fire.

"Just even being out there, you cannot breathe," Baghdatis said. "I just wanted to, you know, puke."