London (AFP) - Roger Federer, Rafael Nadal and Novak Djokovic have hailed the fast-rising next generation of players, saying the future of tennis is "in good hands" ahead of the start of the ATP Finals.
The "Big Three" remain the men to beat at the season-ending tournament in London -- as they were when they first all played together at the event in 2007.
But the eight-man field at the O2 Arena contains a crop of exciting young players who have their eye on smashing the dominance of the old guard.
Germany's Alexander Zverev, who triumphed last year, is joined by Russia's Daniil Medvedev, Stefanos Tsitsipas of Greece and Italy's Matteo Berrettini.
Dominic Thiem of Austria, at 26 slightly older than his young rivals, completes the field at the tournament featuring the year's best players, which starts on Sunday.
Medvedev, 23, has been the trailblazer for the next generation this year, winning two Masters 1000 titles, reaching the final of the US Open and climbing to number four in the world.
Top seed Nadal, 33, who won the French Open and beat Medvedev in the final of the US Open, said it was an exciting moment for tennis.
"That's the normal cycle of life," he said at the pre-tournament press conference on Friday. "It's healthy for the sport and I think they are super good."
"It will be a good rivalry for our sport," he added. "I will be happy hopefully to keep competing with them for a while but then I will be happy to watch it on TV."
Federer, at 38 the veteran in the field, said he was looking forward to watching the rivalries unfold, describing Zverev's win last year in London as a catalyst for the rest of the group.
"What I like about the young guys qualifying for the first time, it goes away from just saying we've got some talented players on the Tour," he said.
"There's a difference between that and actually being a national hero in your country already and being a top 10 player in the world in tennis, which is not easy to achieve."
There has long been speculation over who would step up to challenge Federer, Nadal and Djokovic, who have dominated men's tennis for 15 years.
Djokovic, the 2019 Australian Open winner and Wimbledon champion, said he and his two great rivals were still glad to be in the mix but that the sport was "in good hands".
"These guys are great guys off the court," he said. "They understand and respect the sport. They show good values. Of course we're going to enjoy cheering them on after we're done with our careers."
- 'Fiery' competition -
Tsitsipas, sixth in the world, said the fierce competition between the younger players would drive them on.
"We're all young," said the 21-year-old. "We're just getting started. It's very fiery. We all want to beat one another."
"We appreciate each other's games," he added. "Each one of us plays differently and it's interesting to see so many varieties. We all come in different shapes and sizes. We are the future. We are the ones that will have to fill the stadiums."
Last year's champion, Zverev, 22, said upsets would become more common as the new generation imposed themselves.
"Rafa, Novak and Roger have had a great rivalry for many, many years," he said. "It started from a young age. So hopefully we are the ones, maybe with some other guys as well, with (Canada's) Denis Shapovalov now playing great tennis.
"Some of the young guys are not here yet but I'm sure will arrive very soon."
"I think we'll see those kinds of surprises many more times in the future," he added.
Zverev acknowledged that Federer, Nadal and Djokovic still deserved their place at the pinnacle of the sport.
"I think the next two or three years are very exciting because the other guys are still there and playing the best tennis in the world. They're still better than us," he said.
"That's a true fact because they're the ones who are winning the big tournaments but I think the young guys are improving quite quickly and it's going to be interesting to see how that goes."