I like the idea of genius as derived from genus, and meaning something like a particular kind of excellence, an exemplar of its kind, something's characteristic disposition. One can think of there being not one class of geniuses, but think of it as excellence of a particular kind, in a particular activity, that is related to the intrinsic and highly developed talents or disposition of the genius. So, Rafael Nadal is a clay court genius, because he is a great tennis player, and in addition he puts maximum effort into the kind of tennis that requires it, and his physical, top-spin heavy tennis is ideally suited to the surface.