People always refer to the Big Four of men's tennis, and in profile terms the phrase still applies. Roger Federer and Rafael Nadal still attract just as much attention as Novak Djokovic, while Andy Murray looks likely to finish the year at a career-best No?2 in the world.
Yet if we look at results over the past 18 months, the Big Four have really narrowed right down to the Ultimate One. In that time, Djokovic has won four grand slam titles, while Marin Cilic and Stan Wawrinka have landed one each.
As for Roger, Rafa and Andy? Nul points.
The same is true at the next level down: the nine top-ranking ATP tournaments known as the Masters. Since the summer of 2014, Djokovic has won seven of the 11 titles on offer, while Federer has landed three, Murray two and Jo-Wilfried Tsonga nipped in with a surprise victory in Toronto. This sequence of dominance can be traced back to a single moment. In the summer of 2014, Djokovic's newfound coaching relationship with Boris Becker was on the rocks. For the first time in four years, he had failed to win the Australian Open.
Then, after another frustrating defeat against Nadal at Roland Garros, he found himself in an uncomfortable and unfamiliar position: going into Wimbledon without a major title all season.
For Becker - who had surprised everyone with his appointment during the previous off-season as Djokovic's new head coach - it was time to deliver. Fortunately for the three-time Wimbledon champion, he was about to find himself back on home turf. SW19 is where he produced his own best performances, and where he now lives. As he wrote in his recently published book, Boris Becker's Wimbledon, he knows every inch of the grounds like the back of his hand.
To return to the 2014 tournament, a scratchy Djokovic fought through to the last four of the event by defeating Cilic in a cliff-hanging five-setter.
And then came the kicker, as Becker's book explains: "The next day, the Thursday, Novak asked me to come to the house he was renting in Wimbledon, and we had a long, three-hour conversation over dinner: Jelena [Djokovic's wife], his brother Marko, Novak and me.
"It was after that conversation that I felt I really belonged in the inner circle of Novak Djokovic. He asked me questions about preparation, about Wimbledon, and many personal things ... Afterwards, Novak said the time spent with me during dinner altered his perception towards me ... That evening he made it clear he really wanted Lilly and me to be there [at his wedding]. So I felt our relationship went to a new level that night."
The point is not so much that Becker has turned his charge into a champion single-handed, for Djokovic has been the pre-eminent member of the Big Four since 2011. Even if Becker's book talks about "changes in his game plan and his positioning, maybe on his serve", these are things that Djokovic could probably have worked out for himself, so hard and relentlessly does he train.
Rather, this is about the level of comfort and stability that has now been established in their back-room team. Because before Becker arrived, Djokovic had a problem: his long-term coach and father figure Marian Vajda had reached the point where he no longer wanted to travel a full schedule.
Who could replace such a pivotal figure, a man who had been at Djokovic's side for a decade? The answer, it seems, was established that night over dinner. Djokovic went on to beat Federer in an epic Wimbledon final three days later, and the Djokopoly was under way.
Most close followers of the sport had seen this moment coming. For all his flawless technique, Djokovic had been underachieving for the previous 21/2 years.
Here was a man who arguably has more exquisite control over a tennis ball than anyone in history, yet who had somehow lost five out of his last six grand-slam finals when he walked out on Centre Court that day with Federer.
The physical fragility of the late 2000s, which evaporated after he was diagnosed with gluten intolerance in 2010, had apparently been replaced by mental fragility.
But this is not the sort of man who allows such issues to go unchallenged. Sooner or later, he was going to make the tour pay. In 2015, the tour certainly has paid - a total of $16,706,125 to date.
Who, then, can stop this frighteningly complete athlete? He is as lithe and supple as a panther, which may explain why he so rarely gets injured. And he is motivated by the very achievable target of hunting down Federer's tally of 17 grand slams. (Djokovic has 10 at present.)
One concern for Federer fans must be that there is no-one coming up behind who looks capable of interrupting the Djokovic supremacy. Only one man born after May 22 1987 - the happy day for the Djokovic family - has won a tournament bigger than an ATP 500 event, and that was Cilic at last year's US Open.
Even at that surprising tournament in New York, there was a sense that the world No?1 was below his peak, exhausted by the emotional demands of his marriage in July and the imminent arrival of his firstborn child. A more genuine frustration for Djokovic must have been his defeat to Wawrinka at Roland Garros five months ago - yet that was perhaps the ultimate "lights out" performance from the Swiss. You do not run into many of those in a season.
The worrying truth for the rest of the pack is that Djokovic has grown into a human roadblock. Over the past 18 months, he has shut down all his rivals old and new, and established a 17-4 record against the other members of the so-called "Big Four".
As he goes out today to start his pursuit of a third successive title at the O2 Arena, the Ultimate One looks practically unbeatable.