Jannik Sinner admits 'I got lucky' after Australian Open heat policy sparks comeback from the brink of retirement

Two-time reigning champion Jannik Sinner admits he "got lucky" before pulling off an incredible escape act to reach the fourth round of the Australian Open.

The Italian world No.2 was on the brink of retiring from his match against plucky American underdog Eliot Spizzirri on Saturday.

Sinner struggled to cope with the blazing Melbourne sun, and was suffering cramps in his arms, hands, and legs during a first set which he lost to the world No.85.

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"Just go point by point, dig deep," his coach Darren Cahill could be heard saying as temperatures climbed towards 40 degrees.

Fans and commentators thought it was a very real possibility he would have to call it quits.

"He's getting cramp, his hand just went, his finger just went and his thumb ... he might be in a bit of trouble here," Aussie great Todd Woodbridge said as he watched on from the court-level commentary box.

"I think he might have had some pickle juice in the break just then too. But definitely his thumb just went.

"This is quite troubling."

With extreme heat protocols in place, players were permitted a 10-minute set break during which Sinner received medical attention and drank pickle juice to combat the cramps.

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When the Open's heat index hit level 5 it forced players off the court and the Rod Laver Arena roof to be close.

That took another eight minutes, allowing Sinner time to recover.

When the players came back the Italian won three sets in a row to win the match 4-6, 6-3, 6-4, 6-4 and progress.

"I struggled physically with the heat today, we all saw this," he told Colin Fleming following the match.

"I got lucky with the heat rule, they closed the roof. I took my time and as the time passed I felt better and better.

"It started with the leg, then it got into the arms, so I was cramping a bit all over.

"But this is the sport. I know it's an area I need to improve, which is a positive thing we try to work on every day. I have a great team behind me who is pushing me every day.

"But tennis is a very mental game, I just tried to stay as calm as possible... I'm here to fight, I'm here to play every point in the best possible way."

Woodbridge agreed the No.2 seed was lucky to make it through.

"He just needed the time, and he was incredibly lucky," Woodbridge said on Nine.

"He can thank his lucky stars the heat kept climbing and he had that opportunity to get off court and receive that bit of treatment, get that fluid, the pickle juice in to fight off those cramps.

"A lot of people would have just shaken hands and said 'I'm done', but not him."

Sinner will face his countryman Luciano Darderi in the round of 16 on Monday.