It's up to his knees & ankles. With his will power and talent 16 is doable.
The question I can't answer is can Federer be the GOAT is he can't beat Nadal?
"I used to be a fairly good tennis player." --Rod Laver. Favs: Goran, Gasquet, Klahn, Seppi, Fognini, Dolgopolov, Cornet