Apart from the matches against Sock, Chardy and Wawrinka, Novak seemed really shaky and vulnerable for the last 2 weeks, whereas Roger mostly looked good in Cincinnati.
I think a lot will come down to Novak's serving performance. After the match against Goffin he said, that, though it wasn't an issue anymore, his recent elbow problem caused him to change his technique on serve which had a negative effect on it's quality and I think numbers would back that up. If Novak serves at his usual level for this year I expect a close match with at least one tiebreak, without a noticable improvement in that area Roger should win in two routine sets.
