Jannik Sinner’s exit from the Montreal Masters raised fresh concerns over his fitness and that issue is emerging as the biggest challenge facing the world No 1 as he looks to finish a triumphant 2024 on a high.

Andrey Rublev withstood a second set collapse to overcome Sinner 6-3, 1-6, 6-2 in the quarter-finals of the Canadian Open in Montreal.

The Russian overpowered the defending champion in the early stages to race to the first set, but his momentum was halted by a half-hour rain delay.

The Italian struck back after the break, clinching the second set without facing a single break point.

Rublev turned the tables in the third set however, repelling five break point opportunities in the opening game en route to opening up a 3-0 lead.

He carried that advantage through to ultimately claim the third set and book a spot in the semi-finals.

“It was a really great match of mine. I was playing really well today and I’m happy that I was able to win,” Rublev told the ATP.

“I don’t know, not much to say, because I still for the moment… really I don’t understand what’s happening, because Jannik is an amazing player and he’s playing unbelievable for the past two years.

“I was just hoping that I would be able to fight with him, to show great tennis. In the end, I think we had a lot of great rallies, a lot of long rallies.

“To be able to win this kind of match, of course, it’s always special, and of course I needed those matches, especially after some bad moments this year. So, of course I’m happy.”

The big concern for Sinner appear to be an injury niggle that may be related to the hip problem that hampered him in the build-up to the French Open.

He missed his ‘home’ Masters tournament in Rome due to a hip issue and while he performed well to reach the semi-finals at Roland Garros, he lost out against Carlos Alcaraz after struggling physically in the final stages of that match.

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Sinner then came close to pulling out of Wimbledon in his last eight clash with Daniil Medvedev as he struggled with an illness that turned out to be tonsillitis.

That forced him to miss the Olympic Games and while he showed flashes of his brilliant best in Montreal, this defeat against Rublev threw up fresh doubts about Sinner’s ability to withstand the physical demands of of competing on the tennis tour.

The pressure of entering every tournament he plays as one of the favourites is still a relatively new experience for Sinner and that adds a weight of expectation every time he steps on court.

If his pain on court in Montreal was a recurrence of the hip problem from earlier this year, alarm bells will be ringing for Sinner as he was given medical advice to rest that problem before he made his return at the French Open.

With the US Open just around the corner, his participation in next week’s Cincinnati Masters event must now be in doubt and with hard courts an unforgiving surface, best-of-five-set matches at Flushing Meadows later this month will test Sinner’s physical boundaries to breaking point.

With Alcaraz set to return to ATP Tour action in Cincinnati after his emotional journey at the Paris Olympics and Novak Djokovic likely to play at the US Open without any warm-up events on US hard courts, Sinner’s two primary rivals at Flushing Meadows are nothing heading into the final major of the year with preparations that have been affected by their heroics in Paris.

Yet the players ranked at No 2 and No 3 in the ATP rankings appear to be injury-free and that is the key ingredient for success amid a hectic 2024 tennis schedule.

Who is Jannik Sinner’s biggest opponent?

From the evidence of what we have seen in the opening half of 2024, a body that continues to throw up challenges in front of Sinner appears to be as challenging as any of his opponents.