Novak shall face (big server) Anderson in R2. But, Anderson is currently ranked outside 100. Also, their h2h is 9-2 in favor of Novak. The 2 wins of Anderson are not noteworthy either with one coming in Laver Cup and the other in 2008, when Novak has not established himself. I don't see any chance for an upset in this match (although would like it if it happens).
The only chance of upset is he goes with the Querrey model of 2016, i.e., serve great, and hope Novak goes walkabout. It was (and is to date) only Querrey's 2nd win over Djokovic. They have a few things in common: tall guys with big serves, and veteran enough to face the #1 calmly. Obviously, KA has the advantage of a W final and a USO one, too, so he'll have some belief. If he's in a decent place in his recovery, he does qualify as a dangerous floater. He's got some grass matches under his spikes this year, but I personally doubt it will be enough. Plus, I don't think the similarities to Querrey will not be lost on Novak. I'm fairly sure he'll want to come out hot and not lose the first set like he did today.