Jannik Sinner barely able to walk during comeback win over Eliot Spizzirri at Australian Open

Two-time defending champion Jannik Sinner has battled through extreme heat and to book his spot in the last 16 of the Australian Open.

Sinner was in all sorts of trouble midway through his clash against American Eliot Spizzirri before recovering to seal a remarkable 4-6 6-3 6-4 6-4 victory.

With temperatures soaring into the high 30s, Sinner’s entire body shut down at the start of the third set in extraordinary scenes.

The world No.2 was cramping everywhere and could barely walk at times. He even copped a time violating after physically not being able to get to the line to serve.

He could be heard telling his team he didn’t know what to do.

“Go point by point, mate. Dig deep,” Australian super-coach Darren Cahill urged his charge from the champ’s courtside pod.

“You’ve just got to get through the third set, mate. Even if you walk around. Don’t worry.”

He was broken and trailed 3-1 before the extreme heat rule came into effect and players were told to leave the court so the roof at Rod Laver Arena could be shut.

“He’s getting cramp, his hand just went, his finger just went and his thumb ... he might be in a bit of trouble here,” Todd Woodbridge said on Nine.

“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.”

In extraordinary piece of timing, the extreme heat policy was applied as soon as Sinner dropped serve.

TV footage captured Cahill following tournament director Craig Tiley down the Melbourne Park hallways as the RLA roof was being closed.

Spizzirri looked frustrated by the situation given he was well on top at the time.

“Jannik Sinner has never been more relieved to come off the court,” Brenton Speed said on Nine.

“What an incredible story this is.”

The short seven-minute pause allowed Sinner to have some pickle juice and get some movement back into his body.

After the match, he admitted the heat rule saved him.

“I struggled physically a bit today. I was lucky with the heat rule,” he said.

“You know they close the roof, I took my my time. And as the time passed I felt better and better and I’m very happy about this performance.”

He returned under the closed roof and immediately broke back to get the set back on serve.

Sinner was still appearing to struggle, but stepped it up at the end of the set to break at 4-4 and then hold to love.

Under the heat policy, players were given an extended break between sets, which allowed Sinner more time to recover.

Sinner still appeared in discomfort in the fourth set and dropped his serve early to fall behind 3-1.

But the two-time champion reeled off the next four games before serving out the match.

It was a disappointing end for Spizzirri, who deserves a lot of credit for his performance.

The world No.85 played well above his ranking and may have gone on to win that crucial third set had the heat rule not come into effect.

Nevertheless, the third-round performance is the 24-year-old’s best at grand slam level.

Sinner will now play 22nd seed Luciano Darderi in the round of 16 after the fellow Italian defeated Karen Khachanov in four sets.

And rounding out an historic, if testing, day for Italian tennis, fifth seed Lorenzo Musetti gave the Davis Cup champions three men in the fourth round for the first time with a 6-7 6-4 6-2 5-7 6-2 win Czech Tomas Machac.

“Happy that this finished. We fought really hard. I knew it before starting the match that today with the heat I was prepared to fight ‘til the end,” Musetti said after inflicting the first defeat of the year on Machac.

Musetti will await the winner of 40-year-old former AO champion Stan Wawrinka’s clash with ninth seed Taylor Fritz later on Saturday night.