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The men's draw at Flushing Meadows has been blown wide open by early exits for Alcaraz and Djokovic, but the No.1 seed remains in the hunt after a composed performance.

He reached the fourth round for a fourth consecutive year with a routine 6-1 6-4 6-2 victory, setting the tone with several huge groundstrokes in the opening set.

O'Connell never looked likely to fight back as Sinner smashed 46 winners in a ruthless performance, wrapping up his win in one hour and 56 minutes.

The Australian Open champion will face either Tommy Paul or Gabriel Diallo for a place in the last eight, and he was relieved to survive an opening week full of surprises.

"Everything can happen in this sport, so I try to stay on my side of the net and I'm trying to take the tournament day by day," Sinner said. 

"Each opponent is a very tough challenge and [I am] also enjoying every moment on the court. There have been already a couple of upsets, so let's see what's coming.

"But I'm very happy to still be here and hopefully I can play as many matches as possible. But as I said, we go day by day and then we'll see how it goes."

Sinner lost his first set of the tournament to Mackenzie McDonald but has been largely untouchable since then.

He has only lost 18 games across his past nine sets of tennis, an average of two per set.

The Italian's winning streak now stands at seven straight matches, meanwhile, after his triumphant Cincinnati Open run.