It seems hard to believe, but Federer, who boasts 20 grand slams, said that his 'mindset was completely different' when he was younger.
The Swiss star has set numerous unprecedented records, won countless trophies and earned his millions since he turned professional in 1998.
The 37-year-old, however, recently told Credit Suisse
he hasn't always been this way.
"I was trying to breakthrough on the Tour. Now it's about maintaining a certain fitness level, staying motivated, having different challenges than when I was younger," Federer said.
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"When I was younger everything was new, it's like a kid in a candy store."
Tennis is a gruelling sport and Federer's approaching 21 years as a pro - travelling, training and competing are all elements that are sure to have taken their toll on the eight-time Wimbledon Champion's body.
"It was great or terrible, because I'd be horribly tired from jet lag or I'd be totally unmotivated on some days but I overcame these difficult times," he continued.
"I realised that mental toughness and mental strength was one of the most important factors for a successful career especially for the long-term."
Mental toughness is what Federer credits with his incredible success. He also has some advice for the younger generation looking to show off their talents. - some advice his rivals could use.
"I was able to achieve that [mental strength], so it's one of the things that actually made me most proud," the Swiss star explained.
"A lot of advice I could give is about how to overcome when you are struggling.
"The passion has to be there, the love for the game has to be there. You have to see that travelling should be exciting, packing your bags and going to a place should be great."
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Federer ended this season with a Nitto ATP finals semi-final defeat to Alexander Zverev, but he highlighted the importance of keeping a positive mindset.
He opened the tournament with a shock loss to Japanese star Kei Nishikori, before two wins over South African Kevin Anderson and Belgian Dominic Thiem led him to a match-up with 21-year-old German Zverev - a match he lost 7-5, 7-6(5).
"It shouldn't all be bad and disappointing, you have to see the positives in some tough moments," Federer noted.
"And when you are on the tennis court, you have to to give it your all, you believe in your team, you believe in your game plan and you go for it.
Sometimes you can, sometimes you can't win them all, that's what my parents always told me but you can try your very best and that's what you always should strive for."