NEW YORK — Ask other tennis players what makes Novak Djokovic great, what has pushed him to the brink of the first calendar-year Grand Slam by a man in more than a half-century, and the responses might include a mention of the way he returns serves or his ability to cover the court or his two-handed backhand.
And so on.
What they also invariably praise are his mental strength and physical stamina, his focus and his fitness, especially when it comes to the best-of-five-set format used at the major tournaments where he is 26-0 in 2021 heading into his U.S. Open semifinal against Tokyo Olympics gold medalist Alexander Zverev on Friday.
Novak Djokovic's keys to success
Just a sampling of assessments from male and female pros:
— Sam Querrey: "His best trait is his mind."
— Maria Sakkari: "Novak is from a different planet. Really. I mean, the way he plays, and the way he sees, tennis (is) like no one else."
— Steve Johnson: "His belief is so high."
— Dusan Lajovic: "The persistence and level of consistency that he has is just unquestionably his biggest weapon."
— Jessica Pegula: "I don't see any weaknesses. We've seen him cruise through tournaments. We've seen him come from behind and get himself out of situations. He seems to conserve his energy and know when to turn it up."
The latest to stand across the net from Djokovic was No. 6 seed Matteo Berrettini of Italy in the U.S. Open quarterfinals.
Like Djokovic's previous two opponents at Flushing Meadows, Berrettini grabbed the 77-minute opening set. Like in Djokovic's previous two matches — and six others at Slams this season when he dropped one or two sets to start — he won, as Berrettini faded to a 5-7, 6-2, 6-2, 6-3 defeat.
It brought to mind what former player Andy Roddick, the 2003 U.S. Open champion who went 5-4 against Djokovic from 2007-12 when their careers overlapped, tweeted a couple of days earlier: "First he takes your legs ……. Then he takes your soul."
So which is tougher to overcome against Djokovic, the prowess of his body or his mind?
"A little bit of both. Even if you see him get flustered, he can get ... in his zone. That's something that, over time, he's created. You feel that from the other side of the net," Berrettini said. "From a physical standpoint, I feel like I can play at a high level, but it almost seems like he doesn't get fatigued. It's kind of like he says, 'OK, bring it. Tired? I can stay here for three or four days.' That's the sensation."
Berrettini is all too familiar with that sensation.
He also took the first set on the way to a four-set loss in the Wimbledon final in July, which followed Djokovic's triumphs at the Australian Open in February and the French Open in June.
"You need to keep up that physical and mental intensity that we had in the first set for four or five sets. That's the key. It's not a given that you can't do that, but no one in the world has managed to," Berrettini said, then added with a chuckle: "There are 8 billion of us, and no one has done it."
Now it's Zverev's turn to try.
He enters Friday on a 16-match winning streak, including coming back from a set and a break down to upset Djokovic 1-6, 6-3, 6-1 in the semifinals of the Summer Games on July 30.
"I feel like I was the first player to beat him in a very big match this year. That does give you something," said Zverev, who lost in the 2020 U.S. Open final to Dominic Thiem.
One not-so-insignificant difference between that Olympic match and what awaits Friday: The contest in Japan was best-of-three-sets.
The longer format, everyone agrees, favors Djokovic.
Which is at least part of the reason that if the 34-year-old from Serbia can win against Zverev and win again in the final — where the foe would be No. 2 Daniil Medvedev or No. 12 Felix Auger-Aliassime — Djokovic would become the first man since Rod Laver in 1969 to collect all four major singles tennis championships in one year.
Such a feat requires excellence in match after match, over month after month, on hard courts, clay courts and grass courts, against players of various size and skill.
So how does Djokovic explain what he's been able to do?
"I've worked over the years to perfect my game so that my game can have literally no flaws. Every player has some weaknesses in his game. There's always something you can improve. I want to have as complete of an all-around game as I possibly can, so that when I'm playing someone, I can adjust on any surface, I can come up with different styles of play, I can tactically implement the game that I need for that particular match in order to win," he said.
"Of course, I want my opponents to feel that I can get any ball, that I can play comfortably from back of the court, (at) the net, serving, returning," Djokovic continued. "Over the years, working on perfecting the game has really helped me, I think, just be very adaptable to anybody's game and to any surface."