Defending champion Jannik Sinner was cramping and struggling to move as he trailed unseeded Eliot ​Spizzirri 6-4 3-6 3-1 on Rod Laver Arena.

The tournament's Heat Stress Scale hit 5.0 just as Sinner went down 3-1 in the third set, giving him eight minutes of relief before play resumed.

Fellow Italian Lorenzo Musetti was ‌leading Tomas Machac 2-0 in the fifth set at John Cain Arena when play was paused while Valentin Vacherot ‌and Ben Shelton began their match at Margaret Court Arena with the roof closed.

The tournament referee said matches would not be played on outside courts until 5.30 p.m. (0630 GMT).

Play had started an hour earlier to take advantage of the relative cool of the morning but conditions were already stifling as defending champion Madison Keys entered centre court ⁠for her third-round match at 10:30 a.m..

Used to hot conditions, Florida-based Keys needed only 75 minutes to beat Karolina Pliskova. ‌Compatriots Jessica Pegula and Amanda Anisimova, who also live in Florida, seemed to thrive in the heat as they ​moved on to the next round.

"I didn't have to kind of kill myself today," said Pegula after her quick-fire 6-3 6-2 win over Oksana Selekhmeteva.

Anisimova, who wrapped up her 6-1 6-4 win over fellow American Peyton Stearns in just 71 minutes, said she was prepared for the stifling conditions.

"Just the fact that we had so many outdoor courts in the summer, it was always so hot, I hated training in the summer," ‌she added.

"But you know what? At least it paid off a little bit. It's just great to be playing out here even when it's so hot, it's nice to have a challenge."

The Australian Open operates under an extreme heat policy that considers air temperature, radiant heat, wind speed and humidity to assess ⁠playing conditions, allowing officials to delay or suspend matches when thresholds are exceeded.

The tournament's five-point scale was at 3.4 in the early afternoon when Sinner and Spizzirri arrived on court.

Players were allowed longer breaks between sets when the scale reached four points. Five points means the suspension of matches on outside courts and closing the roofs of the main showcourts.

Organisers warned fans to bring hats, drink plenty of water and make use of misting fans scattered around Melbourne Park.

Sydney resident Sarah Gellatly covered herself in sunscreen from one of the dispensers just outside the Melbourne Park gates.

She said players would be concerned about battling each other during the hottest part of the day.

"It's really hot but if you're a player that's touring around the world every day, then they're used to the heat," she told Reuters.

Melbourne resident Mat Vonarx and ⁠his partner had the cheap "ground pass" tickets which give access to the outside courts and said ‌they were ready to handle whatever the weather threw at them.

"We knew it was going to be very hot today but it's the tennis," Vonarx told Reuters on his first visit to the Grand Slam. "We're ready for it, we've brought plenty of water and we're good to go."

Vonarx said the players would just have to tough it out.

"It's going to be tough for any ⁠sport in this condition but they're professionals, they're going to do what they need to do ​to complete the game," he said.