(spoiler sorry - hope no-one's day is ruined by it)
Well this ends on an odd note for all those who reckoned Wawrinka was going to spank Fed. I think a minority, myself included, could see that a lot of Wawrinka's form was made possible because of the relative kooks he played previously who fed him ideal height, short balls for most of the matches.
As Federer just showed, when you lump on some pressure and play deep more often a player's form can unravel pretty quickly. When you're forced to play the bulk of a match 2m behind the baseline instead of being allowed to stand on it things aren't so dandy.
(Giving him some credit though, he wasn't as good as he's been. How much of that was down to Federer is to be debated. I think it's most of it...)
I think you hit the nail on the head. Fed put Wawrinka on the move, pushed him back and kept the ball out of his wheelhouse for the most part. I do think that nerves did play a part, but Federer outplayed Wawrinka on the backhand side. Along with every other stroke.