Let's start with Karatsev first. The Russian has been really solid this week, basically cruising past all of his opponents. He's not known for consistency, but every now and then, he'll have a good week or two where he seemingly looks unbeatable.
That's been the case this week, as his powerful baseline play is creating all sorts of problems for his opponents. The fast conditions benefit his attacking style of play, and it's worked so far. Shelton has a similarly aggressive approach but he also has a powerful serve.
That's the edge he'll have in this matchup, so as long as he can serve well, Shelton should be able to keep Karatsev away from chances. Fatigue is a worry, though, as he played a lot of matches recently, and we've seen him battle fatigue symptoms for a few matches now.
They played once so far, and it happened at the US Open recently. Shelton dominated the matchup, winning in four sets, and he's not really lost that level since then. He might be tired, but he's never played better tennis and never looked as confident as he's looked at the Japan Open recently.
So, considering all of this, how might the final go? Well, if Karatsev brings his absolute best, I think he'll win simply because Shelton looked all but done against Marcos Giron. A better player like Karatsev could have finished off that match, but Giron didn't and ended up losing.
I don't think we'll see the best of Karatsev though as Shelton is certainly going to frustrate him with his serve if nothing else. The chance to win a maiden ATP trophy should also motivate Shelton so I'll back him to get it done.
Prediction: Ben Shelton to win in two sets.