Melbourne’s heatwave has disrupted the Australian Open for a second time this tournament as the big guns continue to battle indoors.

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.

It could be the hottest day in Melbourne ever, after the city suffered through 46.4 degree heat in 2009.

Usually-busy Garden Square and nearby eateries were almost deserted as the temperature edged towards 40 degrees just before 2pm.

The Heat Stress Scale (HSS) was first 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.

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 4.0 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 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.

The decision means big-serving German Alexander Zverev’s quarter-final clash with American Learner Tien will be held entirely under the roof.

Women’s seeds Coco Gauff and Elina Svitolina open the night session from 7pm, before home hope Alex de Minaur takes on career slam-hunting Carlos Alcaraz.

De Minaur, the sixth seed, isn’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.

“I assume the roof will be closed. I don’t know for certain, but if the heat rule is into effect, it’s going to make it a little bit of an indoor match.

“Hey, it is what it is. I mean, once I rock up on Tuesday and I walk on court, you just deal with whatever’s in front of you.”

Alcaraz, meanwhile, conceded “you’ve got to get used to everything” as he puts the forecast heat out of his mind in preparation for de Minaur.

“Whatever it is, I just will be ready,” Alcaraz said.

“I will be focused. I will try not to think about it.

“I will try not to affect my game at all, and I think that’s it.”

Svitolina has reached the quarter-finals for the second straight year and is eyeing a first appearance in the Open’s last four.

The Ukrainian hopes her later timeslot will grant some relief from the heat.

“It’s of course better to play in the evening. It’s much nicer,” Svitolina said.

“Of course, still the ball is flying quite a bit because of the heat, but yeah, it’s going to be fine either way.”

Officials on Monday confirmed a 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 on Wednesday.

It 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.

-AAP