Iga Swiatek and Daniil Medvedev, the last two former champions left in the U.S. Open singles draws after a series of upsets, swept into the quarter-finals at Flushing Meadows with a minimum of fuss on Monday.

Top seed Swiatek, who won the title two years ago, showcased her all-court game in a straightforward 6-4 6-1 win over Russian Liudmila Samsonova to open the evening session at Arthur Ashe Stadium and warned she was only going to improve.

"At the beginning, it wasn't easy to get the rhythm, especially because we kind of put our focus more on recovery than actually practicing before the slam," Swiatek said of her week in New York.

"So I'm happy that I got some matches here and for sure, I'm feeling better and better every day."

Medvedev, the 2021 champion, had earlier brushed aside a delay caused by a fire alarm in the building where the Hawkeye line-calling system is housed to pummel Portugal's Nuno Borge 6-0 6-1 6-3.

"It was a strange experience but it did help me because I felt like he had started to get the momentum and this broke it a little bit," Medvedev said.

"But that's things that we don't control. Hopefully it never happens again."

The Russian will next face the winner of the final match of the day, a blockbuster fourth round tie between men's top seed Jannik Sinner and local hope Tommy Paul.

There was already one success for Americans to cheer on their Labor Day holiday as Jessica Pegula efficiently disposed of Diana Shnaider 6-4 6-2 to reach the last eight without dropping a set.

The sixth seed, who missed the French Open due to injury but triumphed at a U.S. Open tune-up event in Canada last month, said she felt her game was trending in the right direction.

"Today was the best I've felt off the ground so I want to keep working and bringing my best tennis for the later rounds," the 30-year-old said.

"There's always a lot of pressure here but I like it. It's nice being an American here and getting all the support."

Pegula has reached the quarter-finals of all four Grand Slams but never gone further and her path to the semis this week is blocked by the imposing figure of world number one Swiatek.

PAOLINI OUT

The only minor surprise early on the eighth day of the championships also came in the women's draw when Karolina Muchova, a semi-finalist here last year, ousted Italian fifth seed Jasmine Paolini 6-3 6-3.

Unseeded after surgery on a wrist injury sustained at last year's U.S. Open sidelined her for 10 months, the Czech showed her quality to outclass Paolini, who had reached the finals at both the French Open and Wimbledon this year.

Muchova will next face Beatriz Haddad Maia after the Brazilian produced 40 winners to send former world number one Caroline Wozniacki packing 6-2 3-6 6-3.

Later at the Louis Armstrong Stadium, Jack Draper became the first British man since Andy Murray in 2016 to reach the last eight at Flushing Meadows with a 6-3 6-1 6-2 thrashing of Tomas Machac.

The 25th seed has yet to drop a set at the tournament this year and will face Australian Alex de Minaur in his first Grand Slam quarter-final on Wednesday.

De Minaur came out on top 6-0 3-6 6-3 7-5 in a tight tussle with compatriot and close friend Jordan Thompson to reach the last eight at a third straight Grand Slam.

"I've got so much respect for Thommo," said the 10th seed. "We've grown up together, he's been like a big brother to me, so I really appreciate everything he's done for me, and he's a hell of a competitor."