Nick Kyrgios is a "great, great player" according to Roger Federer, but the Australian has to be willing "to work at it" if he wants to have a successful career.
Federer, who is preparing for his comeback at the Hopman Cup and Australian Open in January after a six-month layoff with a back injury, says he's been impressed with Kyrgios ever since he first trained with him in 2014.
"What I know is he's an exciting player to watch. He's a great shotmaker, got a massive serve and with those capabilities come great potential for great results," Federer told The New York Times.
"The question is, the mind, the body, how much is he willing to work at it? And he seems to check in and out of that sometimes, but ever since he came to Zurich to train with me a month before he beat Rafa [Nadal] at Wimbledon, I've thought he was a great, great player. And he still is going to stay that way for years to come.
"Great player": Nick Kyrgios received high praise from Roger Federer.Credit: Getty Images
"The question is how much does he want it? I think it's all going to boil down to that, and that's going to determine how much success he's going to have."
Kyrgios, who will also begin his 2017 at the Hopman Cup, is returning to competition after serving a ban following his suspension for "conduct contrary to the integrity of the game" during a tournament in Shanghai.
Kyrgios was initially suspended for eight weeks and fined $3200 following his behaviour on court during a 48-minute loss to Mischa Zverev after appearing to put in no effort at various stages of the match – even walking to the change of ends before one of his opponent's serves had landed. Chair umpire Ali Nilli told Kyrgios: "You can't play like that, OK? That's not professional."
His ban was reduced to three weeks after he underwent a number of sessions with a sports psychologist.
Roger Federer says it's important to know where the boundaries are.Credit: Joe Armao
Federer, who is regarded as not only one of the greatest players of all-time but also one of the most respected, said it was important that the next generation of players behaved well on and off the court.
"I think it would be good to have because our game sort of creates that, like golf. We have usually classy players, classy attitudes. But at the same time, I don't believe everybody has to be exactly like that. I like different attitudes, different characters, different talk, different behaviour," Federer said.
"Obviously, if you step outside the box, it's like a kid, you tell them, 'no, you're not allowed to do that'. And that's why you have rules, but you can stretch the rules. I'm cool with that. Nick I'm sure knows what the boundaries are more now because of what happened."
Federer, who along with Nadal, Andy Murray and Novak Djokovic have dominated the men's circuit for more than a decade, said the old guard were looking over their shoulders at the next generation of stars such as Kyrgios, Milos Raonic and Dominic Thiem.
"I think next year we'll know again better who is going to be that next player, and I think it's going to be an exciting year as well with the youngsters and with us coming back, Rafa and me, and Andy and Novak fighting for what they are fighting for.
"I think Raonic is on the doorstep. Can Stan [Wawrinka] win another slam this next year? There are some great storylines. I've always been a big fan of young players coming up and there seems to be some talent there now, which I like to see."