Despite all appearances as he ploughed through the draws at all four grand slams this year, world number one Novak Djokovic does have a breaking point.
Djokovic hit it on Sunday, falling 6-4 6-4 6-4 to Daniil Medvedev in the US Open final as the 20-time major champion finally proved unable to will himself out of an early hole.
It was the fifth successive match in which Djokovic had dropped the opening set at Flushing Meadows, and the Serb superstar acknowledged afterward all of the energy he expended to get to the brink of the first men's calendar Grand Slam –since 1969 and a record 21st major title might finally have caught up with him.
"Could be, could be [time spent on court a factor]. I had more hours on the court spent from Daniil, that's for sure," Djokovic said at his post-match news conference. "But was also emotionally very demanding period for me in the last five, six months. Slams and Olympics and playing at home in Belgrade.
"Everything was coming together for me here and kind of accumulating all the emotions that I've been through.
"Unfortunately I didn't make it in the final step. But when you draw a line, you have to be very satisfied with the year. Three wins, three slams and a final. For the last couple of years I've been very transparent and vocal about my goals, to play my best tennis at slams. I'm managing to do that.
"Of course, I was short today for another slam title, but I have to be proud with everything that my team and I have achieved. And in tennis we learn very quickly how to turn the next page.
"Very soon there are some more challenges, more things that are coming up. I have learned to overcome these kind of tough losses in the finals of slams, the ones that hurt the most."
This defeat sent a range of emotions surging through the emotional 34-year-old as he sat courtside while awaiting the trophy presentation.
Asked what was going through his mind at that moment, Djokovic's initial answer was succinct.
"Relief," he said. "I was glad it was over because the build up for this tournament and everything that mentally, emotionally I had to deal with throughout the tournament in the last couple of weeks was just a lot. It was a lot to handle.
"I was just glad that finally the run is over. At the same time I felt sadness, disappointment, and also gratitude for the crowd and for that special moment that they've created for me on the court."
Djokovic heaped praise on his opponent, saying he expects Medvedev to win more grand slams in the years ahead after the Russian broke through for his first.
That inevitably sparked thoughts of the coming generational change in the men's game, dominated for so long by Djokovic, Rafael Nadal (35) and Roger Federer (40).
With an astonishing 60 grand slam titles between them, their eventual departure from the stage will open up opportunities for Medvedev, Tokyo Olympics gold medallist Alexander Zverev and other younger players.
Djokovic insisted he is not quite done yet, but he does believe that transition already is in progress.
"The older guys are still hanging on," he said. "We're still trying to shine the light on the tennis world as much as we possibly can.
"I'm speaking on my own behalf. I still want to keep going, try to win more slams, play for my country. Those are the things that motivate me the most I think at this point.
"But the new generation, if you want to call them this way, is not anyone new. It's already current, established. Of course, they are going to take over.
"I think tennis is in good hands because they're all nice guys and very, very good, high-quality tennis players. They got something to offer on and off the court.
"We are hoping that the transition will be smooth in terms of the attention and the popularity of this sport. It's very important.
"We all, of course, want to win on the court, but at the same time we all at the top represent this sport. We need to be aware of that, take this responsibility and try to bring more fans to the world of tennis.
"At the end of the day that's what counts and that's what keeps our sport alive."