World number one Jannik Sinner says there is still room for improvement despite recording a 23rd successive match victory in hard-court Grand Slams to reach the US Open third round.

Having lost just four games in a dominant first-round win over Vit Kopriva, defending champion Sinner produced another efficient display against world number 36 Alexei Popyrin.

It was another ominous message to Sinner's rivals - Popyrin, who beat Novak Djokovic in the third round in New York last year, had been seen a potentially tricky opponent for the Italian.

But Sinner broke Popyrin's serve early in each of the three sets and wrapped up a clinical 6-3, 6-2, 6-2 victory on Arthur Ashe Stadium in just over two hours.

Sinner has not lost a match at a hard-court major since the 2023 US Open, when he lost in the fourth round to Alexander Zverev.

He won the title at Flushing Meadows last year and is the two-time defending Australian Open champion.

Wimbledon champion Sinner said he was unhappy with the way he served against Popyrin, having landed only 51% of his first serves.

"I felt like we both didn't serve great but I was returning very well, especially on the second serve," he said.

"I'm very happy about today. Obviously I'm aiming to improve the serve but, about the rest, I feel quite comfortable."

Sinner will face Denis Shapovalov, the 27th seed from Canada, in the third round as he bids for a third major of the year.

Sinner's ruthlessness was demonstrated midway through the second set when, serving at 3-2 up with a break, he saved three break points to secure an important hold.

In the very next game, Sinner broke to love, quashing any hope of a Popyrin fightback.

There was a similar pattern at the start of set three when Popyrin failed to capitalise from 15-40 on Sinner's serve, and the top seed again made his opponent pay with a break to love in the next game.

Majchrzak & Cobolli win five-set epics

Elsewhere, Poland's Kamil Majchrzak produced a superb comeback to knock out Russian ninth seed Karen Khachanov in an epic second-round contest.

Majchrzak came back from two sets down, overturned a 5-2 fifth-set deficit and saved five match points to come through 2-6 6-7 (4-7) 6-4 7-5 7-6 (10-5).

Meanwhile, Russian Andrey Rublev came through a tough four-set encounter with American Tristan Boyer.

Boyer, ranked 113th in the world, extended the match by taking the third set before 15th seed Rublev completed a 6-3 6-3 5-7 7-6 (7-4) victory.

Rublev's next opponent will be Coleman Wong, who backed up his history-making first-round victory against Aleksandar Kovacevic with a four-set win over Australia's Adam Walton.

Wong's first-round success made him the first male player from Hong Kong to win a Grand Slam singles match in the Open era.

Lorenzo Musetti was a 6-4 6-0 6-2 winner over Belgian veteran David Goffin and the 10th seed will face fellow Italian Flavio Cobolli next.

Cobolli, the 24th seed, won a match tie-break to beat American Jenson Brooksby in a marathon second-round match that lasted more than four and a half hours.