Roger Federer believes 2020 will be toughest yet for the Big Three of tennis

Rafael Nadal, Novak Djokovic and Federer all miss out as Dominic Thiem and Stefanos Tsitsipas make the ATP Final in London

Rising star Dominic Thiem after winning his semi-final match against Alexander Zverev at the ATP Finals in London. PA

Austrian Thiem celebrates his victory, which secured him his first appearance in the final of the ATP event. EPA

Thiem signs autographs after winning his semi final, and he takes on Greek 21-year-old Stefanos Tsitsipas in the final on Sunday. EPA

Alexander Zverev, the 22-year-old German, was the defending champion before his defeat to Thiem. Getty

Zverev was beaten by Thiem at the ATP World Tour Finals in the last four clash. EPA

Switzerland's Roger Federer returns against Greece's Stefanos Tsitsipas during their singles semi-final matchat the ATP World Tour Finals in London. AFP

Roger Federer, 38, found it tough going against 21-year-old Greek Stefanos Tsitsipas in their semi-final and lost in straight sets. Reuters

Tsitsipas celebrates making his first ATP Final after beating Roger Federer. Getty

Spaniard Rafael Nadal finishes 2019 as world No 1 despite exiting the ATP Finals in the group stage. Getty

Novak Djokovic of Serbia looks dejected in his final game. Last year's world No 1 was knocked out in the group stage after being beaten by Roger Federer. Getty

Roger Federer believes the Big Three could face their toughest challenge yet from a new generation of tennis stars in 2020.

Federer's season ended with a semi-final loss to Stefanos Tsitsipas at the ATP Finals, having seen Novak Djokovic and Rafael Nadal eliminated in the group stage.

Tsitsipas, a 21-year-old Greek playing at the ATP Finals for the first time, is among a handful of up-and-coming talents looking to finally end the unprecedented era of dominance by the trio.

Other youngsters at the season-ending tournament included defending champion Alexander Zverev, 22, and US Open runner-up Daniil Medvedev, 23.

Dominic Thiem, at 26, made his first ATP Final when he beat Zverev 7-5, 6-3 in the second semi-final on Saturday night.

However, it’s not the first time that the 38-year-old Federer has faced talk about a talented crop of youngsters, even though none of them have so far been able to break the iron grip on the sport.

”It's the same question every year at the end of the year,” said Federer. “But does it feel like this year might be the best year yet? Possibly. But then I look at the list of who finished world No 1, who has been world No 1 all these years, and it's just crazy that it's always one of us.

"But we are not getting any younger. So chances increase not because we are getting worse but because they are getting better.”

Nadal clinched his fifth year-end No 1 title this week, putting him level with both Federer and Djokovic. Since 2004, the only other person to finish the year on top of the rankings was Andy Murray in 2016.

In addition, the Big Three have won the last 12 Grand Slam titles between them. And until one of the youngsters ends that streak, any talk of a generation shift may be premature.

”Definitely it's that next step they need,” Federer said. ”The only issue is that it seems like me, Novak, and Rafa are healthy, healthier than maybe in previous years, as well.”

Zverev, for his part, is convinced that the new guard is ready to compete for majors.

"I think next year will bring a new Grand Slam champion. We'll see who that will be, because I think the young guys are playing incredible tennis," Zverev said after losing to Thiem.

"You know, it can be Daniil, it can be Stefanos, it can be Dominic. I'm in the mix, as well, I hope. We'll see what next year brings. But I think for the young guys, it's going to be an exciting year."

The third-ranked Federer ends the year with four ATP titles, but failed to add to his record haul of 20 Grand Slams when he lost an epic Wimbledon final to Djokovic.

He also fell short of a record-extending seventh ATP Finals title after converting just one of 12 break points against Tsitsipas, losing 6-3, 6-4.

As long as he stays healthy, though, Federer is confident he’s still good enough to compete for more majors next season.

“I've got to keep on playing at the level like I have this year, and then I will create some chances,” he said. “Then when the matches come, it's not as easy as it was maybe 10, 15 years ago where you're just going to play very good, day in, day out.

"Opportunities were there today, they were there in other moments as well this season. That can change an entire season around, the confidence around, the flow of things. But I'm happy how I played this season, and I'm extremely excited for next season.”