Boris Becker believes tennis’s “big three” will have their hold on the sport tested like never before when the next generation descend on Australia in January.

Roger Federer showed there was still plenty left in his 38-year-old tank with a vintage 6-4 6-3 defeat of Novak Djokovic at London’s ATP Finals on Friday morning, keeping him in the hunt for a seventh season-ending crown.

German great Becker was on hand to witness his “phenomenal” performance and still thinks the Swiss star, world No.2 Djokovic and world No.1 Rafael Nadal will be hard to beat over five sets at the Australian Open.

But with Dominic Thiem beating both Federer and Djokovic in London and Daniil Medvedev pushing Nadal to the brink, Becker believes the gap has never been closer.

“We’ve been talking about it for two years, but this week for the first time there’s a couple of players beating the top three,” Becker told AAP.

“As long as Roger is healthy and plays tennis you can’t write him off but (his mental edge on younger opponents) is more challenged every year and they’re getting closer and closer.”

Former world No.1 Becker will captain Germany in the inaugural ATP Cup, to take place from January 3-12 in Brisbane, Sydney and Perth ahead of the year’s first grand slam.

Alexander Zverev will lead Germany against Alex De Minaur, Nick Kyrgios (Australia), Stefanos Tsitsipas (Greece), Denis Shapovalov and Felix Auger-Aliassime (Canada) in a Brisbane group he thinks is the pick of the bunch.

Djokovic’s Serbia will also feature in the other Brisbane group, with Federer the only member of the top 10 not playing the tournament.

“That’s the present and the future; it doesn’t get much better than that,” Becker said of the mix of talent on display in Brisbane.

“Australia on home soil is the ultimate challenge, but Canada and Greece are tough and playing them all back-to-back is something to look forward to.”