NEW YORK • Physical durability is a prime requisite at any of tennis' four Grand Slam events but the cement of the US Open is regarded as the hardest physical test of them all, mainly because it comes after such a demanding time in the tennis calendar.
Andy Murray is hopeful but far from convinced that he is tough enough to withstand the rigours of the next fortnight.
On the face of things, the Briton is currently the fittest of the famed men's Big Four.
Kei Nishikori referred to the Scot as an Iron Man and the Japanese most certainly has a point.
Novak Djokovic, the defending champion, does not know whether his troublesome left wrist, the reason for his shock first-round exit at the Olympic Games, is sufficiently recovered for the tests that await.
He also has to be in a better frame of mind after admitting to personal problems.
"The wrist hasn't been in an ideal state for the past 31/2 weeks," Djokovic said.
"But I'm doing everything in my power with the medical team to make sure that I'm as close to 100 per cent as possible."
Rafael Nadal is equally in the dark. He looked physically exhausted in Cincinnati after missing Wimbledon and followed that up with a draining Olympic schedule in Rio de Janeiro.
Roger Federer, of course, will not play at Flushing Meadows on doctors' orders.
Murray has been relentless over the past four months, reaching the finals of the past seven events he has contested and winning more than he has lost.
However, such excellence may well come at a cost and, after enjoying a couple of days of complete rest following his 22-match winning run being halted by Marin Cilic in the Cincinnati Masters final a week ago, the Scot was not up for any false bravado.
"My body is not fresh," he said.
"I have played an awful lot of tennis, a lot of matches. Often it's been the case at the US Open that guys at this stage of the year are fatigued, especially the ones who have been competing for the Majors throughout the year.
"But do I see a greater opportunity for me because of the doubts around Novak?
"No, I don't think so. I've had a long summer as well.
"Plus Novak and Rafa are in the other side of the draw to me. Them having wrist problems doesn't really affect my chances of getting to the final.
"There are opportunities for other guys to make a breakthrough and win the major events. That's going to happen over the next few years, for sure, because I can't go on forever."
Murray has had a stellar first half of the year, reaching the finals of all three of this year's previous Grand Slams.
Throw in for good measure tournament victories in Rome and Queen's Club and, of course, there is that second Olympic gold medal won in Rio two weeks ago.
Although most of the players Murray would want to avoid - including the likes of Cilic and Wimbledon runner-up Milos Raonic - are in the opposite half of the draw, he is certainly not discounting the more youthful threats posed by an apparently resurgent Grigor Dimitrov, Dominic Thiem, Alexander Zverev and Nishikori.
Naturally, Murray wants to continue adding to his three Grand Slam titles, a tally which still remains small compared to the hauls of Federer (17), Nadal (14) and Djokovic (12).
He has all the potential to claim a second US Open, four years after his breakthrough in New York saw him become the first British man in 76 years to win a Grand Slam singles title.
The only worry for him is his relatively mediocre recent record in New York - runs to the quarter-finals in 2013 and 2014 were followed by a fourth-round exit to Kevin Anderson 12 months ago.
But, at 29, he admits that he is taking positives from being in the twilight of his career.
"You have to make the most of every opportunity. It's a slightly different mentality to maybe when you're younger and you feel like you have a bit more time on your side," he said.
THE TIMES, LONDON, AGENCE FRANCE-PRESSE
US OPEN DAY 1
StarHub Ch201, 11pm