Roger Federer could carry on playing until he reaches 40 "without any issue" according to his former fitness trainer Paul Dorochenko, who revealed that the world number one was tough to deal with during childhood.

Federer recently admitted that he is looking forward to retiring from the sport he has dominated for so long having experienced a glimpse of life after tennis when recovering from a serious knee injury in 2016, but the 20-time Grand Slam champion is not planning to call time on his glistening career just yet.

The legendary Swiss recently reclaimed the world No.1 ranking for the first time in almost five-and-a-half years and is arguably playing some of the best tennis of his career as he approaches his 37th birthday. He may have one eye on retirement, but Dorochenko thinks the eight-time Wimbledon champion will be gracing courts around the world for a few years to come.

"For me, he [Federer] can play until he is 40 without any issue," Dorochenko told La Razon, relayed by Tennis World USA. He will stop when he starts losing to more normal people. He likes to win, like everyone, but he likes it more. Federer has this thing that the "tops" have: [Zinedine] Zidane, [Michael] Jordan...he moves in an impressive way."

Federer's elegant, serene nature both on and off the court has helped garner millions of admirers over the years, but Dorochenko can remember a time when he was anything but the picture of calm he is now.

The fitness trainer and physiotherapist can remember a "crazy" Federer that used to smash rackets and conduct himself poorly and he never imagined the Swiss would enjoy the success he has managed to achieve, but now Dorochenko is backing his former tutee to add to his 20 Grand Slams before he calls time on his illustrious career.

"The truth is that he was a very complicated kid, very active, half crazy," Dorochenko said. "He was a good person, but a very complicated one: was losing rackets, was behaving badly and lost matches.

"His mother always said me that she didn't see him as very smart, and he really never did very well at school but he loves tennis so much and he invented it. I would have never thought that Federer would have been the player of the century, I could not see him winning in Paris, and he did it.

"I knew he could be a great player, that's for sure, but from here to win 20 Grand Slams... and I think he will win more. The good thing for me is that I made a little part of history, and not so many people worked with him."

Roger Federer
Federer is showing no signs of slowing down as he approaches his 37th birthday Getty