Jannik Sinner kept his US Open title defence on track Saturday as Iga Swiatek struggled through to the last 16 and home favourite 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," said Sinner. "The scoreline was a bit against me today, but I just tried to stay there mentally."

World number one Sinner will face either US 14th seed Tommy Paul or Kazakhstan's Alexander Bublik in the next round.

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

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.

"The tournament's still going, the job's not done but this means a lot to me," said Auger-Aliassime, the 25th seed who takes on Andrey Rublev next.

Rublev 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," said Musetti.

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 fights back

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 number two 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.

"I felt like I had nothing to lose because I was losing pretty bad. So in the end I just went for it because what else can you do?" Swiatek said of her fightback.

Russian 13th seed Alexandrova has never made it past the fourth round at a Grand Slam but destroyed Laura Siegemund 6-0, 6-1 and has surrendered just 10 games en route to the second week at Flushing Meadows.

Gauff swept past Polish 28th seed Magdalena Frech 6-3, 6-1 in her most polished display of the tournament to boost her hopes of collecting a second US Open title.

After battling to a tearful victory in the previous round, a much more composed Gauff dispatched Frech with relative ease.

"It's been an emotional week, but I think I needed those tough moments to move forward. I was putting so much pressure on myself," said Gauff, through to the last 16 in New York for the fourth year in a row.

Gauff heads into a showdown on Monday with former world number one Osaka after the Japanese star beat Australian 15th seed Daria Kasatkina 6-0, 4-6, 6-3.

Four-time Grand Slam champion Osaka is enjoying her best US Open showing since winning the tournament for the second time in 2020.

The last-16 match will be a repeat of the duo's first meeting at the US Open in 2019, when Osaka overpowered the then 15-year-old Gauff in straight sets.

"Everyone will be watching. That's what tennis needs," said Venus Williams, who progressed to round three of the the women's doubles on Saturday with partner Leylah Fernandez.

Wimbledon runner-up Amanda Anisimova beat Romania's Jaqueline Cristian 6-4, 4-6, 6-2, while Czech 11th seed Karolina Muchova and Ukrainian 27th seed Marta Kostyuk both won in three sets.