Six-time champion Novak Djokovic says a big-business approach to the Australian Open is testing player welfare to the limit in dangerously hot conditions at Melbourne Park.

Still cooling off after his four-set comeback win against Frenchman Gael Monfils in sweltering 39.9C heat, the 12-time grand slam-winning Serb admitted he had struggled to draw breath at times on Thursday.

“The conditions were brutal,” Djokovic told reporters after stepping off the Rod Laver Arena furnace, with 67C recorded at ground level.

“It was a big challenge for both of us to be on the court, to be able to finish the match.”

Asked whether Open officials should have postponed the match, the 30-year-old said it was a tough call.

“It was right at the limit,” Djokovic said.

“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 it comes down.”

But Djokovic, returning to the tour after a season-ending elbow injury in 2017, understood Tennis Australia’s decision to push ahead with the schedule.

“People might say, ‘well, at this level you have to be as a professional tennis player fit’,” he said.

“But I think 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.”

The superstar Serb, bidding to become the lone man to win seven Norman Brookes trophies, hinted that ticket sales were viewed as more important to tennis bosses.

“You’re a part of the industry,” he said.

“Our sport has become an industry, like most of the other global sports.

“It’s more business than a sport. At times I mind that, I don’t like that.”

Post-match, Monfils said he felt like he was “dying” for 40 minutes, while Germany’s Andrea Petkovic told Reuters heat-related fatigue wore her down mentally.

Players weren’t the only people suffering the summer sun at Melbourne Park.

On show court 3, staff members stepped in to treat a young child, who appeared to be five or six years old, with ice packs suffering in the heat.

Another mother took her three-year-old child out of the stadium to cool down as the heat became too much to bear, AAP understands.

Tennis Australia has been approached for comment on whether its extreme heat policy, twice invoked in 2009 and 2014, will be enforced with the Bureau of Meteorology forecasting a top of 42C on Friday.