Here's another interview from April where he says it very clearly ("Clay, my favourite surface"): http://www.lequipe.fr/Tennis/breves2...-preferee.html «C'est la surface que je préfère. De loin. Quand je me sens bien, c'est la surface où je me sens le plus fort» (= It's the surface I like the most. By far. When I feel well, it's the surface where I feel the strongest.)
He likes hardcourts too, but the quote was about clay.
I don't get why a lot of people assume he would have kicked ass big time if he had played the US hardcourt events.
Remember when he won Indianapolis last year, it was such a struggle, he had to go the distance in each match (except the final), he only made it to the final because Haas flipped his lid and Querrey collapsed physically... His only good match of the week was the final.