This young gun has it all!

'What I like about Zverev is he's got the full package'

Roger Federer has paid a glowing tribute to Alexander Zverev, hailing the 20-year-old German as having the 'full package' of skills and resolve to lead tennis's youthful revolution.

Zverev offered a stern challenge before finally succumbing to Federer, a man 16 years his senior, at the ATP Finals in an absorbing three-setter on Tuesday.

Federer has lost twice to Zverev on tour, and once in the Hopman Cup, and believes the German will be a major threat to the game's elite for years to come -- and perhaps to him again even as early as Sunday.

With one win and one loss at the O2 Arena, Zverev could still feature in the other half of the draw to Federer in Saturday's semi-finals should he prevail in the winner-takes-all clash with Jack Sock on Thursday.

Federer said he was impressed not only with how the world number three plays the game but also with how he is looking further down the line to plan for his future in tennis.

"I like what I'm seeing in 'Sascha,'" Federer told reporters after his 7-6(6), 5-7, 6-1 victory.

"I see somebody who is working towards the future. What I like to see is I feel like he's working towards how he could be playing when he's 23, 24 in terms of his fitness, planning, organisation, all these things."

While mentioning Zverev's name alongside that of 22-year-old Nick Kyrgios and 18-year-old Canadian Denis Shapovalov as youthful players who impressed him, Federer added: "What I like about Zverev is he's got the full package."

"He's already three in the world. I think he's going to leave the Finals here, regardless of whether he qualifies for the semis or not, with a lot of information."

Zverev could not manage to repeat August's Canadian Open victory over an injury-hampered Federer, which stands as the 19-time Grand Slam winner's only defeat in a final in 2017 and his only loss to a fellow top-10 player all year.

"But I think the last six months of the season have given him everything he needs to work forward to," said Federer.

"Then, of course, he's only going to get stronger from here. This should be very encouraging for him and his team."

Only Federer and Rafael Nadal have won more than Zverev's five titles this season and the youngster said he felt, even in defeat, he had taken another significant step forward.

"It was a very positive match for me," said the man from Hamburg.

"I think if I continue having this level, I don't know, maybe you'll see me at the weekend."