Terrassa (Barcelona), November 15: Paying glowing tribute to young tennis sensation Alexander Zverev, Roger Federer has hailed the 20-year-old German as having the "full package” of skills and resolve to lead tennis’ youthful revolution.

Zverev offered a stern challenge before succumbing to the 19-time Grand Slam champion -- a man 16 years his senior -- at the ATP Tour Finals in an absorbing three-setter at the London’s O2 Arena.

Federer, who has lost twice to Zverev on Tour, and once in the Hopman Cup, 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 semifinals 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 No.3 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 (8-6), 5-7, 6-1 victory.

“I see somebody who is working toward the future. What I like to see is I feel like he’s working toward 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 repeat August’s Canadian Open victory over an injury-hampered Federer, which stands as the Swiss ace’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. Then, of course, he’s only going to get stronger from here. This should be very encouraging for him and his team,” feels Federer.

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.”

Federer secured his place in the semifinals for a record 14th time.

Returning the compliment, Zverev said, "He's (Federer) the best player of all-time. "He's always going to find a way to try to disturb your rhythm a little bit and try to win."