Nole will be eager to win in Paris and he is going to be at or near his best in RG. Rafa will be keen to gain the mental edge and no better place than Paris. Outside of these two, Murray will not be able to hurt the top three on clay. I am yet to see him performing great on clay over 5 sets. So I am being bold and discounting Murray here.
Fed plays very very well on clay. Its his best surface (IMO) and he is someone Nole will shudder to face.