Don't forget that Roger had a very tough route in 2017 AO: He had to beat Berdych, Nishikori and Wawrinka (among others) before facing Rafa.
My point being, if a truly great player is on his game, the draw doesn't matter so much. Given that Roger hasn't been on his game since at least February, we were all hoping for the "luck of the draw" and it didn't come through because Novak's in his quarter. But the Roger of last year's AO/Wimbledon would have beaten all but the very best Novak, and maybe even beaten him.
I guess what I'm trying to get at is that if we're hoping for an easy draw for Roger to get #21, we probably will be disappointed. For him to win that 21st Slam he needs to find the form that brought him #s 18-20. And it is possible, I think, at least within the span of a Slam.
That said, I think he's got maybe 4 more chances: this USO and three Slams next year.