Re: When did these players begin their decline in your opinon
federer hasn't been quite the same since his peak years in 2005, 2006 and early 2007.
however, i'd say that in the past 6 months he's been playing the best tennis i've seen of him since 2008. lots of ups and downs have passed since that year, though. i recall federer played almost as well as in his peak years in AO 2010, for example, and then played at his muggiest in the next months.
as for the reasons of roger's slow decline, the speed is the most notorious factor. fed's game depended (and still does) big time on his footwork and that's not quite the same as before. nowadays he makes less adjustment small steps and that in turn makes him more prone to errors and shanks. still, on a good day the differences are only minor...