NEW YORK (AFP) — Jannik Sinner kept his US Open title defense on track Saturday as Iga Swiatek struggled through to the Last 16 and home favorite Coco Gauff booked a blockbuster clash with Naomi Osaka.

Sinner rallied from a set down to beat Canadian 27th seed Denis Shapovalov 5-7, 6-4, 6-3, 6-3 and is trying to become the first man to retain the title in New York since Roger Federer in 2008.

"I was in a very difficult moment," Sinner said.

"The scoreline was a bit against me today, but I just tried to stay there mentally."

World No. 1 Sinner will face Alexander Bublik next after the mercurial Kazakh put out US 14th seed Tommy Paul in a late-night thriller, winning in five sets.

"Week two is completely different. It's getting tougher and tougher," Sinner said.

Third seed Alexander Zverev fell to his earliest US Open exit in seven years at the hands of an inspired Felix Auger-Aliassime.

Canada's Auger-Aliassime belted 50 winners but was a point away from going two sets down before outduelling Zverev 4-6, 7-6 (9/7), 6-4, 6-4.

"I didn't play a good match, and not a good tournament in general," the 2020 runner-up Zverev said.

Auger-Aliassime next takes on Andrey Rublev, who put an end to the fairytale run of Hong Kong's Coleman Wong in five sets.

There were three more retirements in the men's draw on Saturday after the injury-induced exit of sixth seed Ben Shelton the day before.

Alex de Minaur, Lorenzo Musetti and 435th-ranked Swiss qualifier Leandro Riedi all moved on when their opponents threw in the towel.

Eighth seed De Minaur led Daniel Altmaier 6-7 (7/9), 6-3, 6-4, 2-0 when the German retired, while 10th seed Musetti held a two-set lead over Flavio Cobolli before the latter quit because of an arm injury.

"I didn't want to finish like that, especially against Flavio. He's probably one of my best friends on tour," Musetti said.

Riedi became the lowest-ranked man to reach the fourth round of a major since 2002 as Polish opponent Kamil Majchrzak retired after eight games due to injury.

Swiatek showed great resolve to dig herself out of trouble against Russian 29th seed Anna Kalinskaya under the lights of Arthur Ashe Stadium.

The world No. 2 slid 5-1 behind in the opening set but saved four set points and won a tie-break before eventually seeing off Kalinskaya 7-6 (7/2), 6-4.

Swiatek, the 2022 US Open champion and six-time major winner, was far from her best but scrambled through to a Last-16 meeting with Ekaterina Alexandrova.