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.
Play on outside courts did not resume until after 7.30pm and the roof at Rod Laver Arena was finally opened partially after Elina Svitolina's 6-1 6-2 demolition of out-of-sorts American Coco Gauff.
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.
By 9pm, the mercury had dropped to 29C as a long-awaited cool change swept through the city.
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 indoors throughout.
It played right into the big-serving German's hands as he banged down a whopping 24 aces and 56 winners.
"With the roof on, it's perfect conditions," Zverev said.
"It's a very solid 24C with AC on ... I would love to play with the roof every single match."
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.
30°C