Portugal Colonial - Struggling Sinner drops set before making Melbourne last 16

The two-time defending champion was broken three times in the first set and six times during the match as he laboured to a 4-6, 6-3, 6-4,6-4 victory on Rod Laver Arena over a gruelling 3hr 45min.

His reward is a clash next will fellow Italian Luciano Darderi, the 22nd seed.

With temperatures climbing to a forecast 40C, it was not the demolition job many predicted against a player ranked 85 with Sinner struggling to acclimatise in the brutal conditions.

Draping himself in ice towels during the changeovers, he suffered a fright in the third set with severe cramp that needed treatment.

Hobbling, he was only saved when play was halted due to the extreme heat and the stadium roof closed.

It was the first time Sinner had dropped a set in 12 matches dating back to Vienna last year against Alexander Zverev.

"I struggled physically a bit today as you saw. I got lucky with the heat rule, they closed the roof," he said.

"As the time passed, I felt better and better, and very happy about this performance.

"Looking back, in every big tournament I've had some really tough matches. So hopefully this can give me some positives for the next round."

Sinner is striving to win three Melbourne titles in a row and join Novak Djokovic as the only men to do so in the Open era (since 1968).

He came out firing, holding serve then securing an immediate break before inexplicably letting his guard down to be broken back to love.

The imperious Italian got in his groove and broke again with a searing crosscourt forehand winner for 4-2, but once again was broken to love.

Sinner struggled in the heat, broken for a third time to slide 5-4 behind with a composed Spizzirri serving out the set in 43 minutes.

Incredibly, Spizzirri broke again for 1-0 in the second set, leaving Sinner shaking his head.

The world number two slowly began to figure out the American and broke back and held for 2-1. With momentum back in his favour he made no mistakes to take the set.

But the energy-sapping weather took its toll and Sinner was left limping after a 20-shot rally in the third set.

He was in real trouble and slumped 3-1 behind before the extreme heat rule was applied and he left the court while the roof was closing.

When they resumed, he broke straight away, looking more comfortable out of the sun's glare, with another break leaving him serving out for the set.

Unfazed, Spizzirri kept applying pressure and the pair exchanged breaks in the fourth set before Sinner finally seized control.

F.Cardoso--PC