Listen Federer was good enough to win slams between 2010 and 2019. He did win a couple. He just didn’t win enough to maybe get 23 slams.If you think physical aspect doesn't lead to an impact on mental game, I don't know what to say.
I see Fed and Djoko mental strength on a similar level considering career, Djoko a little better, but not by too much.
Granted 11 Djoker is probably the toughest anyone has been mentally in a year or a span of 9 months.