The Australian Open has copped criticism from outraged fans for its heat policy as players were forced to play in temperatures approaching 40 degrees.

Conditions in the middle of the court at Rod Laver Arena were even recorded at 69C, where six-time champion Novak Djokovic defeated French star Gael Monfils.

Read more: When should sport be postponed or halted due to heat?

Monfils sensationally told a chair umpire he was about to collapse as he struggled to deal with the Melbourne heat.

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The French star pleaded with the chair umpire to allow him more time in between points as he visibly struggled in the sweltering heat.

"If I can't take longer than 25 seconds between points, I am going to collapse," Monfils said during a change of ends.

"I'm sick to the stomach. I'm tired and dizzy."

Monfils played on but regularly complained to the chair umpire and tournament officials, asking for longer breaks in between points.

His request was denied because the wet bulb used to measure when play should be suspended had not hit the 32.5C mark.

After winning the first set, Monfils deteriorated in the heat and went on to lose to the former world number one 4-6 6-3 6-1 6-3.

Australian Open officials defended their extreme heat policy after receiving a barrage of complaints from fans on social media.

"The health of our players is of paramount concern, but we need to be consistent with the outside courts so some don't get an unfair advantage," the tournament tweeted.

"The referee will initiate the Extreme Heat Policy once the ambient temperature exceeds 40C & the Wet Bulb index (WBGT) exceeds 32.5C."

The extreme heat at the open has been a hotly debated topic all week as temperatures have soared close to 40C at Melbourne Park.

Dr Liz Hanna of the Climate Change Institute at the Australian National University and the Climate and Health Alliance told news.com.au "someone could die" if players pushed their bodies too far in extreme conditions.

"(The temperatures players face) are a lot hotter than recorded and we know with tennis players - particularly when they're playing at their full capacity - their muscles are generating an enormous amount of additional heat," she told news.com.au.

"They're at very high risk of overheating and that is potentially lethal. The worst case scenario is that somebody would succumb to severe injury."

The Australian Open has been no stranger to extreme heat throughout the years.

In 2016, players were taken off the courts during the opening round of qualifiers, while Canadian Frank Dancevic complained about the open's "inhuman" conditions in 2014.