MELBOURNE (AFP) - The heat-baked Australian Open has been hit by a record number of pull-outs, organisers said on Wednesday, Jan 15, 2014, as temperatures hovered around 40 deg C for a second straight day.

After a first round overshadowed by the sight of players fainting and vomiting in the heat, nine players either retired or forfeited their matches, according to an official tally.

The total is the highest in one round at the Australian Open and equals the Grand Slam Open-era record, matching the first round at the 2011 US Open and Wimbledon's second round last year.

Although most of the players cited injuries, Dutch player Robin Haase went out with cramp, which is often set off by high heat.

Canada's Frank Dancevic, who fainted during his loss to Benoit Paire, said it was easier for players to suffer injuries when affected by the heat.

"I think when you're running around on court quite dizzy, you're more prone to getting injured also," Dancevic said. "You can't play the way you want to play, you can't move the way you want to.

"For sure, it can make players have injuries."

It is also difficult to quantify how much players weighed the difficult conditions as a factor when deciding to call it quits.

Slovenia's Polona Hercog raised eyebrows when she retired with a rib injury after winning just one point against Alize Cornet. All players received US$30,000 (S$38,000) for contesting the first round.

Playing conditions remain in sharp focus with temperatures forecast to return to 41 deg C on Wednesday and stay high on Thursday and Friday before cooler weather sets in.

Dancevic blasted the conditions as "inhumane", while Britain's Andy Murray warned organisers were risking a serious incident by letting play continue.

"Whether it's safe or not, I don't know. You've just got to be very careful these days," said the Wimbledon champion.

"There's been some issues in other sports with, you know, players having heart attacks. I don't know exactly why that is. Or collapsing."

Tournament referee Wayne McKewen said while conditions were "hot and uncomfortable" they were not dangerous because humidity remained low.

And chief medical officer Tim Wood said: "Of course there were a few players who experienced heat-related illness or discomfort, but none required significant medical intervention after they had completed their match."

Also among Tuesday's sufferers was China's Peng Shuai, who vomited and cramped up during her first-round loss to Kurumi Nara of Japan.