They call it stormy Monday – except in the literature for the ATP Tour Finals, which labelled day two “momentous” – and so it proved, calamitously so, for Daniil Medvedev and Rafael Nadal.

The fourth iteration of the season-ending tournament, 50 years after it began in an art museum in Tokyo, has a year to run in London to close out 11 years of extraordinary revenue churning before it moves to Turin in 2021. And, although there have been some low-key performances in recent years at the fag-end of the longest season in sport, the event is winding down on the southern reaches of the Thames in a stylish swirl of surprises and quite lovely tennis.

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First to fall on day one was Roger Federer, who has won a record six titles; then on Monday afternoon, Stefanos Tsitsipas, the Greek prodigy with poetry on his racket, beat his bete noire, Medvedev, for the first time in six attempts, 7-6 (5), 6-4.

In the evening, there was a bigger shot to the system when Alexander Zverev, the youngest defending champion in a decade, marmalised the world No 1, Rafael Nadal, 6-2, 6-4 in an hour and 24 minutes. It was his first win over the Spaniard in six attempts, as well. There was symmetry at work, perhaps.

Nobody expected this, though, despite the crushing fact that Nadal, 33, has not won the title in 15 visits, seven of them aborted through injury. He desperately wanted to play the tournament on clay, but it never happened for him – nor will it in Turin.

Nadal, who said recent abdominal muscle strains did not constrict him, added: “He served huge, very fast. Difficult to read. The physical issue was not an excuse at all. The only excuse is I was not good enough tonight.”

Zverev has much to look forward to. He has beaten Federer (in the semi-final last year), Novak Djokovic (in the final) and Nadal in sequence at this tournament, a statement of intent unlikely to be matched by any of his contemporaries.

Will Tuesday be just as bad for the Swiss? Federer, ground down by Dominic Thiem on Sunday night, has a last shot to fill his boots before embarking on a lucrative exhibition tour when he plays the Italian debutant, Matteo Berrettini, in the afternoon, before Djokovic, who smashed Berrettini, takes on Dominic Thiem in the evening.

It’s a bewildering cross-fertilisation of tennis talent and circumstances: the old hanging on against the new, the new daring to tweak the nose of the old, the very rich protesting they have given enough already to the sport before cashing in their chips in far-flung places. But, at the furnace, life remains very hot.

Tsitsipas described his breakthrough win as: “One of the toughest and one of the most important victories of my career so far. I gave myself a big boost today, kept fighting. That last game was one of the toughest I have played. It wasn’t easy. Such a relief. It is not easy coming knowing you have lost five times before. Much love here. It almost feels like I’m playing in Athens.”

Facebook Twitter Pinterest Daniil Medvedev stretches to play a backhand against Stefanos Tsitsipas. Photograph: Julian Finney/Getty Images

There was a dispiriting sense of ennui in the night match. By the time Zverev stepped up to serve it out, it was tempting to wonder if Nadal wanted to be here in the first place. He has never been comfortable on the indoor hardcourt, which he always regarded as Federer’s preserve, rightly so.

Zverev ploughed through his final service game with admirable coolness, sparing no time for sentiment. The finish was clinical. With two match points in his pocket, he fired in a body serve that Nadal could do no more than bang long.

When he won the final last year, the crowd booed boorishly – and were correctly chided on court by Annabel Croft. This time, they stood to applaud. He beamed broadly as he turned to his audience and said: “It was obviously great. This means everything to me. I was looking forward to it. The goal at the start of the season is to play here in London.”

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As for Tsitsipas, an equally emotional character, his response to victory that carries him further towards the concluding weekend of the season was: “Goose bumps. I used to watch it on TV, dreaming of playing on this court one day. Such an important event. Pure joy playing here. It’s been a long way. It means a lot to me. Things aren’t easy.”

Of the 26 players to beat each of Federer, Nadal and Djokovic, the 21-year-old Tsitsipas is the youngest. Is he the best? That is the intriguing conundrum. They are all brilliant, the young pretenders. Over the next few days they will be testing themselves against each other. It will not be conclusive but it will fuel the absorbing debate about the next generation.

When this tournament landed in London a decade ago, four of the eight finalists were 23 or under: Juan Martín Del Potro, Djokovic, Nadal and Andy Murray. For the first time since, there is a similarly youthful quartet: Tsitsipas, Medvedev (23), Zverev (22) and Berrettini (23). Their chances of emulating their predecessors may be long but they will give it a good shake in the years to come.