Nadal is just extremely profficient on clay. He is a class act. I wanna see him go down, but he won't. I definitely don't think Federer will do well. He had the longest drought from winning a title since he became world number 1.
Just clay. It's the surface that Roland Garros is of course.
He is still ultimately the best player, but clay doesn't appreciate the "winner shots". It's easier to get around, because the balls are slower, and bounce higher.