Interesting comments from Lendl, as a fan back in the day, though don't recall his final couple of years that well. (not as much media outlets back then for following the whole tour year round)I remember Lendl making a comment about why he retired. he said it wasnt a decline in strength or speed that caused his game to drop off as he got older but it was his reaction time. i guess he was saying that he could hit a ball as hard as before (which is not surprising as strength stays longer) and he could sprint as quickly as before (or at least not so far off it was not a critical issue) but it was his ability to respond immediately that fell away and with that goes your ability to get to the ball in position in time against bigger hitters/servers. I would love to see his return of serve stats over time bc if it was his reaction times dropping then that would be the easiest way to spot it.
It's like some other sports, like cricket I mentioned back in 2014, a fraction of a drop off in reflexes and reaction time, and this could be the difference between getting out or clubbing the ball to the boundary.
I recall Adam Gilchrist retired at 36, due to a slight drop in his wicket keeping reflexes. (although his batting was still ok)
SO it is very small margins.
Ironic, we now have a 36.5 y.o. Number 1, so keeping those reflexes must not be the same for everyone.
Of course Federer is not the normal player, most of the current crop will be dropping well down the rankings by then, if they are still around.