The hottest topic of the Australian Open is once again the heat after a day in which Melbourne sweltered and players just about got roasted on court.

He may have been afforded the relative comfort of night-time tennis, but the perma-cool Roger Federer's message differed from some of his cohorts.

"I honestly can't complain — the ATP is doing a nice job and there's a lot of physios and communication going on from officials towards us players," the chilled out Swiss said after his comfortable 6-4, 6-4, 7-6 (7-4) win over 55th-ranked German Jan-Lennard Struff.

"You do know when you come to Australia the heat can sometimes be problematic but everyone faces similar issues.

"Sure I was watching the other players suffer but as long as nothing bad happens it's all good."

Federer may have barely broken a sweat but that match was played at just over 30 degrees Celsius.

Those playing in afternoon matches battled extreme conditions as well as their opponents.

A court temperature reading of 69C was recorded during the match between Novak Djokovic and Gael Monfils, which the Serb eventually won in four sets over nearly three excruciating hours.

Sorry, this video has expired Tennis commentator Peter Mercardo says the argument on whether players should stop playing in the hot temperatures of Melbourne Park is a complicated one. (Image: Reuters/Thomas Peter)

At time both players could be seen slumped over between points, attempting to summon enough energy to continue as the ambient temperature peaked at 40C.

Both received time warnings as they sat drooped in their chairs at change-of-ends, ice packs around their necks.

Djokovic described it as "brutal", while Monfils, whose game dropped away after he had won the first set, said it was risky to play under such conditions.

"It was tough to breathe," Monfils said. "I think it was the hardest I have [experienced].

"For sure, we took a risk. No matter how much you train in the heat, how much you like the heat, it is very tough."

Let's hope the Frenchman isn't familiar with the "sous-vide" cooking technique, which involves slow-heating food such as eggs at temperatures in the mid-60s.

"It was some harm. I get super dizzy," he said.

"I think I have a small heatstroke for 40 minutes … even with the ice towel, the water, I think my body was super warm."

Sorry, this video has expired Aus Open defends event heat policy

The Australian Open's heat policy states matches on the outside courts will be halted and roofs closed on capable venues when the mercury hits 40C (ambient temperature, not court temperature) and the wet-bulb reading gets above 32.5C.

Federer says he expects it to get hot when he comes to Australia. ( AP: Vincent Thian )

A top of 42C is forecast for Melbourne today, and Djokovic indicated that should be enough to postpone matches.

After the match he hinted tournament organisers might have money on their minds, rather than the wellbeing of players.

"There is a limit and that is a level of, I guess, tolerance between being fit and being, I think, in danger in terms of health," Djokovic later said.

"[Today] was right at the limit.

"I think there are certain days where you just have to, as a tournament supervisor, recognise that you might need to give players few extra hours until [the temperature] comes down."

"I understand there is a factor of tickets. If you don't play matches, people will be unhappy."

"It was obvious we both suffered on the court today."

Federer, though, was not so sure.

"On a day like today, what do you do? Do you stop all matches?" the icy 19-time grand slam winner said.

"Lucky guys on the big courts, do they get to play under the roof? Do the other guys get postponed to the next day and is that great?"