It's interesting the bookies do have it close.
Personally Nadal will have to step it up. Medvedev may have lost the last 2 meetings against Nadal but he's improved and has a relative surface advantage. I'm going with Medvedev.
Nadal didn't play a great match against Zverev, obviously, but Medvedev was poor versus Tsitsipas. Since the Greek didn't play particularly well either; well below Zverev's level, in fact, I would say that Nadal looked better than the Russian so far. But, no doubt, both looked a bit deflated.
The question is who wants it more and can step up a notch. I guess since the year-end number one is up for grabs, Nadal should be the more motivated player and will take this unless he gets injured again.
Dull will have to up his level to win this match but considering his caliber, there’s a good chance of that happening. Medvedev doesn’t really have any big shots to hurt Nadal with so as along as Dull doesn’t play as shitty as he did against Doublefaultrev, he should win this match.