The mercury was predicted to soar to 45°C in Melbourne today, with organisers implementing a raft of schedule changes and safety rules to battle the extreme heat.
It could be the hottest day in Melbourne ever, after the city suffered through 46.4°C heat in 2009 - the highest temperature recorded for the Victorian capital.
Women's world No 1 Aryna Sabalenka and Iva Jovic played the opening match on Rod Laver Arena, where the roof was open at the 11.30am start (local time).
Crucially, that would also allow play to begin on outdoor courts under the current heat policy.
But play suspensions were certain later in the day as officials monitor the Heat Stress Scale (HSS) - the system implemented 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.
Play on the outside courts and roof closures on the main arenas would follow the Open's extreme heat protocols.
Australia's Bureau of Meteorology said the hottest part of the day would be 5pm, when the mercury was set to hit 45°C. Severe to extreme heatwave conditions were forecast to peak today and tomorrow, before gradually easing later in the week.
Sabalenka's quarter-final clash with Jovic would be followed by a last-eight battle between men's seeds Alexander Zverev and Learner Tien.
Women's seeds Coco Gauff and Elina Svitolina open the night session from 7pm, when the temperature was forecast to be 43°C, before home hope Alex de Minaur takes on career slam-hunting Carlos Alcaraz.
De Minaur, the sixth seed, was not 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.
Officials yesterday confirmed the series of measures to combat the heat, including allowing ground pass ticket holders access to Margaret Court Arena.
Ballkids will have reduced rotations, increased recovery times, and additional relief squads. And the AO wheelchair championships have been postponed and will start tomorrow.
The move comes after officials were forced to implement extreme heat protocols on Saturday, when play was suspended on outdoor courts and roofs were closed on the major venues.
Sizzling Sinner powers on
One champion survived and another succumbed at Melbourne Park on Monday, as Jannik Sinner reached the quarter-finals after women's holder Madison Keys crashed out at the hands of friend Jessica Pegula.
Sinner became the second Italian into the last eight after Lorenzo Musetti beat an injury-hampered Taylor Fritz 6-2 7-5 6-4, while Iga Swiatek sent the last Australian out of the women's draw with a 6-0 6-3 thumping of qualifier Maddison Inglis.
Two days after suffering dramatic cramps in a third round scare against Eliot Spizzirri, Sinner rebounded with a 6-1 6-3 7-6(2) win over fellow Italian Luciano Darderi as the heat returned to Melbourne Park.
With the sun having retreated in the late afternoon, the double-defending champion was relieved to play in the shade at Margaret Court Arena on a day when temperatures reached 33°C.
"It's definitely a bit different when there is no sun, even if it's quite warm outside still," said Sinner, who booked his ninth successive Grand Slam quarter-final. So I felt quite good out there physically. Everything was okay today."
Sinner will next meet American eighth seed Ben Shelton, who stepped in for the absent Novak Djokovic and overhauled 12th seed Casper Ruud 3-6 6-4 6-3 6-4 in the late match at Rod Laver Arena.
Shelton lost to Sinner in last year's semi-finals, one of a run of eight successive defeats to the Italian.
Djokovic had a walkover when fourth round opponent Jakub Mensik withdrew with injury.
Pegula reached her fourth quarter-final at Melbourne Park and will meet Amanda Anisimova, who made it four US players in the women's final eight after beating China's Wang Xinyu 7-6(4) 6-4.
Second seed Swiatek will face former Wimbledon champion Elena Rybakina in the other quarter-final in the bottom half of the draw, the resurgent Kazakh having blitzed Elise Mertens 6-1 6-3.
- AAP and Reuters
