Definitely the most underrated part in him. Haters keep insisting on his talent, that he was "born with it" (implying he's lucky and deosn't really deserve his success, compared to less talented players that work their ass off and still aren't that successful). Fed wouldn't be "Roger Federer - GOAT" without a true and smart dedication to every aspect of his game, and fitness is definitely a huge part of it, and Pierre Paganini and him have done wonders in the past and are still looking for improvements (despite their already amazing success).
well said JM, Fed might be one of the most hard working players out there, this is very rare among "talented" players, just look at Gasquet (the party boy) or Nalbandian (more burgers please) and the like, for example after MC he had 4 hours of extensive training and practicing and physical conditioning EVERY DAY until Rome, not to mention the unbelievable work and preparations he puts in the off-season.
there is a reason why Federer is able to outlast his opponents and never be tired, and no it isn't genes.