France's world number 42 Alize Cornet suffered dizzy spells and needed a medical examination during her third round loss to Belgian Elise Mertens, and a ripple of concern went through the Hisense Arena crowd when she slumped to the court after serving.
Tournament organisers declined to invoke their heat policy (EHP), saying required thresholds had not been reached to close the roofs of the main stadiums and suspend outdoor matches.
The 27-year-old Cornet said the thresholds needed to be lowered out of respect for players' safety.
"Playing in this condition is of course very dangerous for the health of the player," said the former world number 11.
"The limit of not playing the match is really high, it's like it needs to be above 45 degrees and humidity.
"I think this limit should be a little lower because playing in this condition is not nice for anyone."
Tournament referee Wayne McKewen said in a statement that organisers had "come close" to invoking the EHP but a humidity measure was marginally short of what was required.
Tournament director Craig Tiley said organisers would "consult with the playing group" about the EHP after the grand slam finished.
Petra Martic, who sweated through a 6-3 3-6 7-5 win over Thailand's Luksika Kumkhum, said she had taken pain killers after the second set to deal with blisters on her feet due to the heat coming through her shoes from the roasting court surface at Rod Laver Arena.
"But of course, I mean, these conditions are really, really tough, and there are some girls that don't handle it well. They are having tough times out there," said the 81st-ranked Croatian.
"That was really ugly ... It's really tough on your feet to play in these conditions."
She added that she had hoped organisers could close the roof but heard the temperature was two degrees lower than required.
HEAT SICKNESS
Fifteen-year-old qualifier Marta Kostyuk of Ukraine was also exhausted after her two-set defeat to fourth seeded compatriot Elina Svitolina.
"Like if outside is like 40 (degrees Celsius), you know, 39, and on court, the surface is for sure 70," she said.
"I was going down with the energy more and more with every match."
Fans coming into the tennis centre were warned to wear hats and keep their fluids up to ward off heat sickness even as Briton Kyle Edmund overcame Georgia's Nikoloz Basilashvili in a brutal three-and-a-half hour contest to reach the fourth round.
Tournament organisers were criticised on social media for failing to call a halt to play in a city where outdoor construction workers often down tools for health and safety reasons when temperatures push past 35 degrees Celsius.
Despite the conditions, officials have been strict about enforcing the 25-second time limit between points.
"Let’s pick indoor events to finally give players a dose of what consistent time rules look like!" former U.S. professional and ESPN broadcaster Pam Shriver tweeted.
Whole rows of seats exposed to the sun remained empty at the stadiums as fans draped in wet towels retreated to shady spots around Melbourne Park.
Relief arrived with a cool change that blew over the city late in the afternoon and pushed the temperature back down.
(Editing by Amlan Chakraborty and Nick Mulvenney)
