Federer secured his place in a record 14th ATP World Tour Finals tonight with a three-set win over 20-year-old Alexander Zverev.
The German, a World Tour Finals debutant, was just five when Federer played at this tournament for the first time.
And the Swiss says his attitude to days off such as the one he will take tomorrow has changed significantly in the intervening period.
“I probably didn’t do that back in 2005 in Shanghai during [my first] World Tour Finals,” Federer said.
“I would probably go practice somewhere, somehow, just because you’re supposed to be hitting.
“But I know my game’s not going to disappear if I take a day off. “Actually, it’s quite nice to be able to do that, and having the confidence in your level of play that you can actually keep that up.
“I wasn’t even thinking about taking a day off tomorrow, but Severin and Ivan told me, ‘You should take a day off.’”
He added: “It’s definitely different to 15 years ago.
“I would go play squash sometimes on my off days.
“Then you’re like, ‘Hmm? Wonder why you’re so tired in the semis. How come you have a groin problem all of a sudden? Playing too much soccer maybe on the grass.’”
Federer has since become a father to two sets of twins and also bagged 19 Grand Slam titles.
But he also revealed that a bout of mononucleosis, a viral infection that often results in tiredness, caused him to change his lifestyle.
“I’m not skiing any more like I used to until 2008,” Federer said.
“So, yes, especially with sports, honestly, I’ve cut back completely since 2008, since I had the mono.
“But then the kids keep me busy. I don’t have one, I don’t have two, I have four.
“I don’t want to be just lying on my couch, as well.
“But they also have to be a little bit careful that sometimes I don’t overdo it.
“I like my time with my kids, and I can’t control myself other than just to play with them, as well, and go outside.”