Meet the future stars of men's tennis...

'Competition is very important and it's quite interesting to see so many varieties. All different shapes and sizes'

The greatest trio ever bestowed on men's tennis steadfastly refuse to step aside but this year's ATP Finals boasts a fresh new look that offers a glimpse of the rivalries that should sustain the sport when they go.

Top seed Rafael Nadal, who can clinch the year-end top ranking for the fifth time, was asked if he felt a little old as he sat alongside debutants Daniil Medvedev and Stefanos Tsitsipas, and 2018 champion Alexander Zverev, for Friday's pre-tournament news conference.

And for good reason.

Nadal, 33, will be very much the elder statesman in the Andre Agassi group which starts on Monday. Russian Medvedev is 23, Zverev is 22 and Tsitsipas is 21.

Italian Matteo Berrettini, 23, is the third player making his debut and will have the chance to make an immediate impact when he takes on five-time tournament champion Novak Djokovic when play begins in Bjorn Borg group on Sunday.

While 19-time Grand Slam champion Nadal has injury worries, few would bet against either him, 20-time Grand Slam champion Roger Federer or Djokovic, winner of 16 majors, lifting the trophy next Sunday. But times clearly are changing and the fearless new pretenders are knocking loudly on the door.

A year ago Greek Tsitsipas won the ATP Next Gen Finals in Milan -- a tournament to showcase the rising talents.

He has enjoyed a stunning year, beginning at the Australian Open where he beat Federer on the way to the semi-finals.

Tsitsipas has also beaten Nadal and Djokovic this season and arrives in London ranked sixth. His opening match will be against U.S. Open runner-up Medvedev, who has four titles and a Tour-leading 59 match wins this year.

Asked about the prospect of their future battles, Tsitsipas said: "I guess you could say we are just getting started," he told reporters at the 02 Arena. "We are the future of the ATP."

"Competition is very important and it's quite interesting to see so many varieties. All different shapes and sizes."

Germany's Zverev was the first of the young guns to make inroads, qualifying for the ATP Finals in 2017.

He won it last year and while Grand Slam success remains elusive, he looks set to be battling for the sport's most prestigious silverware with Medvedev and Tsitsipas along with the likes of Canadian duo Denis Shapovalov and Felix Auger Aliassime and Australian Alex de Minaur, to name but a few.

"Obviously, Rafa, Roger and Novak have been at the top for many years and they started from a young age," Zverev said.

"We are the ones now but there are others playing great tennis and there will be a very strong group of guys.

"The next two or three years will be very exciting because the top guys are still there and they are still better than us because they win all the big titles."

Federer, still showing no sign of losing his shine at 38, welcomes the new challenges as he sets about trying to win the season-ender for the first time since 2011.

"What I like about these guys qualifying for the first time is that it goes away from saying we have some talented players coming up on the Tour," the Swiss said.

"They are actually national heroes already in their countries and in the top 10 which is not easy to achieve.

"It's a great experience for them to be here, handling the pressure from the get-go."