Players and spectators sweltered through 39 degree conditions at Melbourne Park, with 42 degrees the forecast for Friday.

Some clever kids took refuge in the water fountains at Garden Square to beat the heat. Outside courts with shade had spectators sitting out of the sun while courts with no shade were empty.

The crowd of 38,072 was down about 10,000 people on the two previous day-time totals.

Most players looked uncomfortable, but Frenchman Gael Monfils battled the most. He lost to Novak Djokovic on Rod Laver Arena where the on court temperature was 69 degrees. He won the first set but then crumbled, virtually giving up in some points, to lose in four sets.

President of Climate and Health Alliance and honorary senior fellow at the ANU's college of science, Dr Liz Hanna, said that spectator and player safety should be taken more seriously at the event.

"If the organisers of the Australian Open insist that it stay in January, then [they must] modify those facilities to keep spectators safe, because otherwise people won't go, or hopefully people will be sensible enough to realise that it will be very risky for their health if they do go," Dr Hanna said.

She suggested more efforts to cool down facilities such as "shade over spectator areas, realising that the sun moves, capacity for wind tunnels, using passive cooling and air movement, and using water spray misters."

The Australian Open has an extreme heat policy. It is enacted when the temperature exceeds 40C and the wet bulb globe temperature rises past 32.5C. Around Melbourne Park there are also fans that blow cold wind and water mist into spectators, and there are free water stations.

Matches on outside courts do not start when the extreme heat policy is in place, while matches in progress are stopped. On indoor courts the match is stopped, the roof is closed and then play resumes.

Some outside courts have shade over parts of the stands while others do not. There is minimal shade for players and spectators on outside courts.

Queensland University of Technology associate professor Adrian Barnett - a researcher who examines heatwaves and public health - said he didn't think the Australian Open should be moved. But he warned people to monitor themselves and others in hot temperatures, not just children and the elderly.

Melbourne University lecturer in the school of building and design Chris Jensen said there were ways stadiums could be built to keep athletes and spectators cooler.

"The best way is getting sun off players, fans and the stadium," Mr Jensen said. "Choosing light colours where possible is helpful."

"Air movement is important ... if Rod Laver Arena had a cross-flow opening on two sides you'd have a noticeable breeze pass through."

Mr Jensen also said using heavy-set materials such as concrete, incorporating plants and greenery into the structure and providing as much shade as possible - for outside courts particularly - were other aspects to include for cooler stadiums.

In 2015, the renovation of Margaret Court Arena won an award for its design, some of which helped make it cooler inside.

with Gene Efron