"He is like a boxer with an uppercut," said Holger Rune's coach Patrick Mouratoglou on Sunday night, after his 19-year-old charge had completed one of the season's most dramatic upsets. "He can use it at any time."
Mouratoglou was talking about the way that Rune - an athletic and baby-faced Dane - can abruptly inject violence into a rally that is apparently going nowhere. "His pace is pom, pom, pom, and then pom - he speeds up all of a sudden." This was the attribute that allowed Rune to outhit Novak Djokovic - still the world's most feared player at 35 - in Sunday's final of the Paris Masters.
Rune's 3-6, 6-3, 7-5 comeback win resonated around the sport like the tocsin that announced the dawn of the French Revolution. Because the ancien regime - that group of elite players that was once known as the Big Four - seem in ever-greater danger of being swept away.
Two of the legendary Four - Roger Federer and Andy Murray - have already vacated their posts because their bodies eventually cried "enough" (even if Murray is still trying to defy medical science as he battles on with his metal hip).
As for the other two, Nadal made a brief visit to Paris, three weeks after the birth of his first child, but fell in the first round to unheralded American Tommy Paul. The most imposing member of the group is still Djokovic - at least, when he is not ruling himself out of events because of his anti-vaxx policy on Covid.
Djokovic takes pride in stifling young talents as soon as they rise up to challenge him, which is why Sunday's defeat is likely to sting. Many more nights like this one, and his locker-room power - which is still a significant component of his success - will start to be compromised.
Djokovic was not present in New York when Carlos Alcaraz - another 19-year-old - lifted the recent US Open trophy. But Alcaraz and Rune, who were born just six days apart in the spring of 2003, stand at the forefront of an exciting young generation that also includes Britain's Jack Draper.
"Rune has better backhand, Alcaraz has better forehand," said Djokovic after Sunday's final. "But they are both improving. They are very complete players for 19-year-olds. Also their energy on the court - motivating themselves and wanting to do well and staying mentally present, it's impressive."
There is always a danger of being carried away by youthful promise. Tennis's camp followers have cried wolf before, when talking up emerging talents such as Daniil Medvedev or Alexander Zverev. We naively thought that these towering physical specimens would grow into mental giants as well. But they have yet to topple their idols. (Medvedev's victory in last year's US Open final, when Djokovic ran out of emotional fuel as he chased the calendar grand slam, feels like the exception that proves the rule.)
Will this new brood finally break through the glass ceiling? As well as having time on their side, they are also unburdened by scar tissue. This was a point that John McEnroe made a few years ago, when he prophesied that the generation born in the early 1990s (Grigor Dimitrov, Milos Raonic etc) would be too brutalised by the superiority of their seniors to realise their full potential.
Now it seems that McEnroe was even more right than we realised at the time. The late 1990s cohort (Medvedev, Zverev, Stefanos Tsitsipas, Dominic Thiem) have also been stifled by the unexpected resurgence of Djokovic, Nadal and Federer over the last six seasons (a period in which these three men won 20 of the available 23 majors.)
Happily, the latest up-and-comers seem to have personality on their side. Alcaraz might not be a great talker, but he invigorates crowds with his electric foot-speed and palpable desperation to track down every ball. As for Rune, he has an intriguing habit of getting into rows, most notably when he claimed to have been yelled at by the mild-mannered Casper Ruud after their French Open quarter-final in June.
Above all, these youngsters play an all-out brand of attacking tennis which is both visceral and spectacular. If their own internal rivalries prove to be as punchy as their strokeplay, we may not miss the fading grandees as badly as we had feared.