Alexei Popyrin's promising start to the Italian Open was quickly rendered irrelevant by the world No.1 who thrashed the 60-ranked Sydneysider 6-2 6-0 in 65 minutes.
Popyrin did not play poorly, but nor did he serve well, and he needed to to have any chance against a home favourite who last lost in February.
That defeat in Doha was against Jakub Mensik, the 26-seeded rising star Popyrin had beaten in the previous round. That followed a first round victory over local favourite and former top-six player Matteo Berrettini.
But any confidence engendered by those victories soon drained away as Sinner began in imperious form.
Popyrin was in trouble within four minutes of play beginning under sunshine and clear skies on the main Campo Centrale court.
Serving first 30-15 suddenly became 30-40, at which stage the Australian double-faulted to concede an immediate break.
Popyrin had won the duo's only previous meeting on clay, at the Madrid Masters, but that was in 2021 when the Italian was still in his teens. Sinner had beaten him twice on hard courts since.
Having saved a break point in his next service game Popyrin, the only Australian of either gender to reach the last 32 and extend their stay in the Italian capital to a second week, had the chance to break back.
But on break point his attempted passing shot just caught the top of the net with Sinner stranded.
That was as good as it got. Popyrin did not force another break point as Sinner went on to extend his run of unbroken service games to 45.
To test Sinner Popyrin needed his serve firing, but in windy conditions he never reached 50 per cent of first serves in and double-faulted again to hand Sinner a 4-1 lead. While Popyrin kept battling Sinner wrapped up the set in 39 minutes.
The second set was even more one-sided, to the extent the home crowd began cheering Popyrin's successes. Popyrin won five points, one of them on Sinner's serve, and lasted 26 minutes.
"It has been a very good performance from my side," said Sinner, whose winning streak is now 25 matches.
"It was very tricky conditions, he is a good server but his percentage wasn't very high, which helped me."
Sinner now meets Italian qualifier Andrea Pellegrino who upset 20th-seed Frances Taifoe 7-6 (8-6) 6-1. Pellegrino, a 29-year-old, has never been in the top 100 and is currently ranked 155, the third-lowest-ranked man to reach the last 16 in Rome this century.
"It is amazing," Pellegrino, who is making his masters debut, said. "When I came here at the beginning of the tournament, I didn't expect to have this kind of result. I am so happy."
Sinner and Pellegrino have met once before, in the final of an ITF event in 2019. Sinner, then aged 17, won 6-1 6-1.
Another American was also surprised as Georgia's Nikoloz Basilashvili beat Brandon Nakashima, seeded 30, 7-6 (7-5) 6-4.
Andrey Rublev, the 12th seed, eased past Alejandro Davidovich Fokina 6-4 6-4 while seventh-seed Daniil Medvedev rallied to overcome Pablo llamas Ruiz 3-6 6-4 6-2.
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