I think you three distinct tiers: first, Rafa as the clear favorite; second, you have three contenders in Novak, Thiem, and Stan, who have separated themselves from the rest of the pack; third, everyone else, from dark horses like Murray, Zverev, Nishikori, Goffin, Tsonga, etc, on down. I suppose you could separate that third tier into dark horses and snowballs (chances in hell), but the point is that anyone other than the top four winning it would be a huge surprise, whether it is Zverev, Cilic, or Pablo Carreno Busta.
If Andy were playing better he'd be in the contenders category, as would Roger if he were playing. But Andy probably has less of a chance of winning this tournament than Zverev, Glassikori, and Goffin, maybe one or two others.