2006 Cincinatti Masters.
Let's also remember that was Roger's prime year
and the only other player to beat him that year was Rafa.
Yes I saw a comment somewhere (might have been Facebook) yesterday that was very complimentary of Andy and his current play but also said he would not have beaten Federer in his prime. I immediately thought of this match which I watched some of recently and wondered if it counted as Federer's prime. I didn't reply to the poster as I didn't want to come off as harsh when they were being nice about Andy.