LONDON • The greatest trio 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 eventually retire.

World No. 1 Rafael Nadal was asked if he felt a little old as he sat beside defending champion Alexander Zverev and debutants Daniil Medvedev and Stefanos Tsitsipas.

At 33, the top-seeded Spaniard will be very much the elder statesman in the Andre Agassi group, which starts tomorrow. Russian Medvedev is 23, Germany's Zverev is 22 and Greek Tsitsipas is 21.

Italy's Matteo Berrettini, 23, is the third player making his debut and the US Open semi-finalist will have the chance to make an immediate impact when he takes on second-ranked and winner of 16 Grand Slam titles, Novak Djokovic, 32, in the Bjorn Borg group today.

While 19-time Slam champion Nadal is facing a race against time to be fit for his opener with Zverev tomorrow, few would bet against either him, Djokovic, or 20-time Slam winner and Roger Federer of Switzerland, lifting the trophy next Sunday.

The Big Three remain the men to beat at the season finale - as they were when they first played together at the event in 2007.

But times clearly are changing and the new pretenders have their eye on smashing the dominance of the old guard in London.

A year ago, Tsitsipas won the ATP Next Gen Finals, an event to showcase the rising talents.

He has enjoyed a fine year since, progressing to the last four of the Australian Open, and he has also beaten Federer, Nadal and Djokovic this season.

The world No. 6's opening match tomorrow will be against US Open runner-up Medvedev, who has four ATP titles and a Tour-leading 59 match wins this season.

Asked about the prospect of future battles, Tsitsipas said: "You could say we're just getting started. We are the future of the ATP."

Zverev was the first of the young guns to make inroads, qualifying for the Finals in 2017 before winning the season finale last year and while Grand Slam success remains elusive, he also looks set to be in the mix for trophies for years to come.

Although the world No. 7 conceded the "Big Three" are "still better than us because they win all the big titles", he predicted "the next two or three years will be very exciting (as) there will be a very strong group of guys (next in line)".

It is a challenge that Federer, Nadal and Djokovic welcome.

Third-ranked Federer, 38, who is aiming to win the Finals for the first time since 2011, believes the young guns have already shown they are more than just "some talented players coming up on the Tour".

Nadal said: "It's healthy for the sport and they are super good. It will be a good rivalry for our sport. I will be happy, hopefully, to keep competing with them for a while."

The Finals is the sole prize that is missing from his trophy-laden career, though he admitted he was not serving at full tilt as he gears up for Zverev.

The French and US Open champion, who had to pull out of the Paris Masters semi-final due to injury, said: "I need to see how things evolve every single day.

"I have confidence that it can go well, but I also have the doubt that it can go badly because the time from Paris until today is very short."

Should he falter in his attempt to win the Finals for the first time in London, then Djokovic is waiting to pounce.

The Serb can claim a record-equalling sixth year-end No. 1 to match American great Pete Sampras if he triumphs, which will be "one of the two biggest achievements that you can have as a professional competitive tennis player".

Djokovic added: "At this stage of my career, in terms of goals, obviously, that's right at the top."

AGENCE FRANCE-PRESSE, REUTERS

ATP FINALS Day 1: StarHub Ch201, 10pm & tomorrow, 4am