Djokovic has scaled down his playing time considerably over recent seasons, tailoring his schedule to focus on the majors.

Moving clear of Australia's Margaret Court in terms of Grand Slam titles is the biggest ambition left in his mind.

His body is not, however, complying.

Djokovic might have defied logic to reach the semi-finals of all four majors this year, but a straight-set defeat by 22-year-old Carlos Alcaraz at Flushing Meadows was another example of his waning physical powers.

Alcaraz and 24-year-old Jannik Sinner have cleaned up the past seven major titles between them, having taken their games to a far superior level than anybody else on the ATP Tour.

"I can do only as much as I can do," Djokovic said.

"It will be very difficult for me in the future to overcome the hurdle of Sinner and Alcaraz in best-of-fives at Grand Slams.

"I think I have a better chance in best-of-three, but best-of-five, it's tough.

"I'm not giving up on Grand Slams in that regard. I'm going to continue fighting and trying to get to the finals and fight for another trophy at least."

Any conversation about tennis' greatest athletes must include Djokovic.

After suffering physical issues early in his career, he transformed himself into a complete athlete and rose to the top of the sport thanks to his speed, strength, stamina and flexibility.

Many have described Djokovic's on-court contortions as "super-human". But he isn't. Father Time is clearly catching up with him like everyone else.

Throughout the whole US Open fortnight he has battled niggles, although he stressed he was not injured against Alcaraz.

"Pretty much every match there is something happening that you have to address and deal with," Djokovic said.

The effort needed to edge points against Alcaraz - especially in a tight second set which you felt he needed to win to stand any chance of turning the match around - guzzled too much fuel.

After winning a 27-shot exchange at 4-4 0-30 down, Djokovic winced and gasped for air before bending over his towel box.

A second wind allowed him to hold serve and take the second set into a tie-break. Once he lost that, he could not recover.

"I ran out of gas. The level of fitness wasn't enough for Carlos," Djokovic admitted.

Djokovic courageously beat Alcaraz in the Australian Open quarter-finals in January, despite being hampered by injury.

But the exertions meant he was not able to complete the semi-final against Alexander Zverev three days later.

He reached the last four at the French Open and Wimbledon but faded again after being unable to break down Sinner.

"It's impressive what he has done this year in the Slams and challenging us the way he's doing," said Alcaraz.

"I told him he looks 25 years old physically. To maintain that level at 38, it's impressive."

Despite his advancing years and scaled-down schedule, Djokovic is still the world's third-best men's player.

There were glimpses of his quality against Alcaraz, such as a vintage backhand winner down the line in the first set, as well as the ball-striking on his way to edging the longest rally of the match in the second.

This level was enough to beat German world number three Zverev and American fourth seed Taylor Fritz in the French Open and US Open quarter-finals.

Ultimately, Djokovic does not quite have the speed or endurance to sustain his highest level against Alcaraz and Sinner.

Yet his Grand Slam performances mean he trails only Alcaraz and Sinner in the annual ATP Tour race - despite playing 11 tournaments this season.