We love to compare athletes from different eras, and ask if Ferenc Puskas was better than Cristiano Ronaldo, Sachin Tendulkar than Donald Bradman, Jesse Owens than Usain Bolt. It's an amusing but useless pastime: a bit like asking how Henry VIII would have got on at the last British General Election.
There are two groups at the ATP Tour Finals event that starts in London on Sunday. One is named for Stan Smith and the other for Illie Nastase. Smith won the inaugural event in 1970; Nastase won it four times, the last in 1975. How would they cope against Novak Djokovic, the current undisputed world No.1?
The answer is that Djokovic would double-bagel them both. Well, maybe Smith would win a game on his serve. But Nastase -- brilliant, beautiful Nastase -- would be blasted off the court. Not because he and Smith were no good, but because the game has changed. Moved on.
Djokovic is probably the finest player who has ever played. That is to say, if the players from former eras were to take him on with the weapons they possessed at their peak, they would all lose.
It's not a fair comparison, though. Djokovic has made himself into the force of the 21st century. He is what tennis has become. John McEnroe, when asked what he did to keep fit, answered: "I play tennis." Good enough for then, not good enough for now.
Djokovic rose to the top in an era dominated by the rivalry between Rafael Nadal and Roger Federer. He has moved on to eclipse them both: his ascent was their decline.
In his group in London are Federer, Tomas Berdych and Kei Nishikori. Djokovic has already brushed aside Nishikori and, on Tuesday, is due to take on Federer.
The competition he faces in his group looks soft; but then all groups look like that for Djokovic right now. It's his game, his world.
He has 10 grand slam singles titles, the usual measure for tennis excellence; Federer has 17 and Nadal 14. But Djokovic looks certain to add to this total. He won 3 this year. He has advanced the game, as great players do.
He reached the top by starting out with great skills: he has gone beyond them all by allying his talent to unstinting commitment.
Being a champion is a lifestyle option. "I turned off the head, but I never turned off the body," Martina Navratilova said in an interview last year, and she revolutionised tennis with her own level of physical commitment. Djokovic has followed that lead and taken it still further.
He was always a very good player with a very committed approach, but two things took him to a level beyond.
The first was his part in Serbia's victory in the Davis Cup in 2010. From that he learned that he could take on anyone in the world and win. He was on a level with Nadal and Federer.
The second was meeting a fellow Serb, Dr Igor Cetojevic, an expert in diet and lifestyle. Djokovic had been prone to asthma, with a consequent lack of stamina in long matches. After the meeting Djokovic changed his diet radically.
He cut out gluten and lactose from his diet, a hard thing for a boy who grew up in his parents' restaurant, one that specialised in pizza and pancakes.
He added all kinds of "super-foods" to his diet; it seems that sea algae is the food of champions. No more cold water, that can affect digestion. It has to be room temperature.
This change in diet had two effects: the first in direct physical benefits, the second in mental posture. With the diet he now knew that everything in his life was directed towards winning. Being a champion was not a job, it was the life he had chosen.
"He's a pro's pro," said Navratilova. "Perhaps that's why he doesn't get as much love as he should." That's an interesting point. In tennis, individuals are feted and sentimentalised more than in any other sport.
Crowd favourites often have two characteristics: breathtaking ability and that loveable touch of vulnerability. Thus we feted the beauty of Federer's play and were equally compelled when he had no answer to the untiring aggression of Nadal.
Nadal's unceasing effort won him a million admirers but his perplexity when things went wrong won him profound sympathy.
Djokovic has neither brilliance nor weakness. He has no perfect stroke, like the Federer forehand or the Nadal topspin. He hasn't got any 10 out of 10s in his armoury, but he hasn't got any 3s or 4s either.
Everything he offers to an opponent is an 8. And right now no one has an answer to that.
The conventional reading of Djokovic is that he was a joker who decided to get serious. It's completely wrong. He doesn't imitate the serving action of Maria Sharapova any more, more's the pity, because of criticism from the humourless. His old friend and Davis Cup colleague Viktor Troicki says that Djokovic is as much of a fun guy as ever. Just not on court.
"These days Djokovic looks grim and implacable from the other side of the net." Simon Barnes
These days he looks grim and implacable from the other side of the net. A calendar year Grand Slam is not beyond him, though that needs phenomenal luck on several levels.
Djokovic only missed out in Paris last year, and there he beat Nadal and Murray on his way to the final, where Stan Wawrinka hit top form.
We look at the limitations of the human body and think that sport can go no further -- only for the athletes to prove everybody wrong.
Right now it's clear that Djokovic is the ultimate tennis player, but perhaps when they're making the draw for the ATP Final of 2050 in the Novak Djokovic group, there will be players who make him look slow, weak, pedestrian -- insufficiently committed.
It's an alarming thought: what more could the players of the future manage to do? For it's a certainty that they will find something. Meanwhile, Djokovic stands before us the most effective tennis player in history. For now.