NEW YORK, Aug 31, 2025 (BSS/AFP) - Reigning champion Jannik Sinner insisted he was "not a machine" after coming through a testing encounter with Denis Shapovalov on Saturday to book his place in the US Open last 16.

The world number one rallied from a set down to beat Shapovalov 5-7, 6-4, 6-3, 6-3 and keep alive his bid to become the first man to retain the title in New York since Roger Federer in 2008.

"A very tough match today. I've known Denis for quite a long time. I knew that I had to play at a very high level," Sinner said after a three-hour battle with the 27th seed from Canada.

The Italian avenged his loss to Shapovalov at the 2021 Australian Open in their only previous meeting.

"I'm not a machine, you know," said Sinner. "I also struggle sometimes.

"Today I felt like from my side I didn't struggle. I was playing great tennis, and he (was) also."

Sinner awaits the winner of the evening session match between US 14th seed Tommy Paul and Alexander Bublik, the 23rd seed from Kazakhstan.

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

"It's a very long way to say that you can defend a title. Now I can say that I'm in the second week, which is already a great result, and then we see what comes next.

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

Sinner made serene progress through the first two rounds but dropped his first set of the tournament against a resurgent Shapovalov, a former top-10 player whose career was stalled by injury.

A US Open quarter-finalist in 2020, Shapovalov signalled his intent with a break in the fourth game and raced 5-2 ahead in the first set.

Sinner broke to get back on serve but Shapovalov produced a gutsy hold, saving a break point for 6-5 before taking the next game and the set.

Top seed Sinner punished a slack service game from Shapovalov at 3-3 in the second set as he dragged himself back into the match at a set apiece.

Shapovalov responded by breaking early in the third set and surging into a 3-0 lead, but he couldn't sustain his level and a defiant Sinner hit back by winning nine games on the spin.

The Canadian offered some belated resistance but by then it was too late as Sinner sealed his 24th successive Grand Slam match win on hard courts.

Sinner has reached the final of all three Grand Slams this season, winning the Australian Open and Wimbledon either side of a defeat to Carlos Alcaraz at the French Open.

Federer is the last man to successfully defend the US Open, winning the title each year from 2004-08.