Monfils began to really suffer during the second set, repeatedly doubling over and not even attempting to return serve during one game.

The Frenchman rated the conditions the toughest he has ever played in, saying: "For sure, we took a risk. I got super dizzy. I think I had a small heat-stroke for 40 minutes. I tried to cool down. But even with the ice towel, the water, I think my body was super warm."

The Australian Open has an extreme heat policy but it does not come into effect until the temperature hits 40C, while a decision on whether play continues also depends on the humidity.

Monfils argued there should be leeway for players to take longer between points than the allotted 25 seconds and for longer breaks between sets when conditions are so physically challenging.

And Djokovic, who is reportedly behind the idea of forming a separate players’ union, made a plea for the health of the players to be put first.

He said: "You work and train hard to be able to sustain these kind of conditions, to be tough. But I think there is a limit, and that is a level of tolerance between being fit and being in danger in terms of health. It was right at the limit.

"Our sport has become an industry, like most of the other global sports. It’s more business than a sport. At times, I don’t like that. What is most important for us is our health and what happens after our career, after you’re 30, 35. There are many players that are struggling."

While Monfils struggled with the conditions, Djokovic fought to try to find his game. He exceeded expectations after six months sidelined by an elbow problem in his first-round clash with Donald Young but this was a much more patchy performance.

Djokovic had trouble with his remodelled service action, serving 11 double faults, and in the first set in particular made a succession of uncharacteristic mistakes.

He said: "I had a nervous start. I wasn’t really comfortable at the very beginning. I can’t blame conditions for my double faults. It’s still that motion that I’m kind of getting used to.

"Being rusty at the beginning is something that you can also expect. I just have to accept it, embrace it, obviously hope for a better day tomorrow and next match."

By the time Djokovic plays Albert Ramos-Vinolas on Saturday the temperature is forecast to have dropped substantially, but those players competing on Friday face even more brutal conditions.

"Honestly, good luck for the guys," said Monfils. "I trained this winter in Miami. It was pretty hot. I thought I was very good. I’m telling you, I was dying on the court for 40 minutes. Sometimes we put our body at risk. Just be smart. If you have to give up, it’s not a shame."

Fifth seed Dominic Thiem showed tremendous mental and physical resilience to defy the conditions and come from two sets down to beat Denis Kudla 6-7 (6/8) 3-6 6-3 6-2 6-3.

There was a big shock on Show Court 2, where seventh seed David Goffin was beaten 1-6 7-6 (7/5) 6-1 7-6 (7/4) by French veteran Julien Benneteau, who is playing at Melbourne Park for the final time.

Juan Martin del Potro was also tested but defeated Russian Karen Khachanov 6-4 7-6 (7/4) 6-7 (0/7) 6-4.

PA