Organisers suspended play on all outside courts at Melbourne Park just before 1.30pm on Tuesday after determining conditions had reached level five on the Open's heat stress scale.
The roofs of Rod Laver and Margaret Court arenas were closed so some singles and doubles action could continue.
Melbourne came close to its hottest day on record of 46.4C in 2009, hitting a maximum of 42.7C after a forecast top of 45C.
The usually-busy Garden Square and eateries were almost deserted as temperature in the precinct hit 40C at 2.30pm.
Despite the heat, 21,226 spectators came through the gates for the sweltering day session after record crowds earlier in the tournament.
Many patrons used misting fans to cool down and sought shelter at Margaret Court Arena, with ground pass holders granted access to the ticketed venue.
An 11.30am start on Rod Laver Arena allowed women's world No.1 Aryna Sabalenka and Iva Jovic's quarter-final to take place with the roof open.
With the heat index passing four in the second set, players would have been allowed a 10-minute cooling break between sets.
But it wasn't needed as Sabalenka swept aside the wilting American 6-0 6-3 in less than 90 minutes.
The roof was closed about 1.15pm during the four-time major champion's on-court interview.
Sabalenka was grateful the roof was not fully open throughout the encounter so she could seek refuge in the shade between points.
"I knew going into this match that they won't let us play on crazy heat," she told reporters.
German third seed Alexander Zverev's 6-3 6-7 (5-7) 6-1 7-6 (7-3) quarter-final win over American Learner Tien was played entirely under the roof.
The roof remained closed as women's seeds Coco Gauff and Elina Svitolina kicked off the night session soon after 7pm.
A cool change was not expected to arrive until late on Tuesday evening, with the mercury tipped to still be at 41C at 8pm and 35C at 9pm.
It raised the prospect of home hope Alex de Minaur locking horns with career slam-hunting Carlos Alcaraz indoors following the women's clash.
Sixth seed de Minaur wasn't bothered by the forecast temperatures for his blockbuster clash with the Spanish world No.1.
"I mean, I'm Aussie. I don't mind the heat," the 26-year-old said.
"I have said it since day one. That's just what I have grown up with, and I'm ready for these types of conditions."
Alcaraz, meanwhile, conceded "you've got to get used to everything" as he put the forecast heat out of his mind in preparation for de Minaur.
"Whatever it is, I just will be ready," Alcaraz said.
The Heat Stress Scale (HSS) was first implemented at the Australian Open in 2019 amid concerns over player and spectator safety.
Four climate factors - air temperature, radiant heat, humidity and wind speed - are measured from various locations at Melbourne Park to determine the HSS measure.
Officials on Monday pre-emptively confirmed a series of measures to combat the heat, including more frequent rotations for ballkids, increased recovery times and additional relief squads.
The start of the wheelchair championships was postponed to Wednesday, when Melbourne is forecast for a much cooler top of 24C.
35°C