In theory Thiem is going to win, but last year Novak also played an average match against him and somehow they went to a third set tiebreak, I hope he raise the level, at least for being competitive, the normal thing would be Thiem winning, but don't rule out a Novak surprise
Thiem should be a favourite here, though obviously you can't write off Djokovic. To me Djokovic has only looked 4th best so far in this tournament, behind Medvedev, Thiem, and probably even Nadal. Just not enough intent and penetration in his shots, bad shot selection, serve slightly shaky. Unless he raises his level, Thiem is going to take full advantage of all the attackable balls and he's probably not going to falter in important moments like Zverev.
It's pretty interesting though, having Nadal and Djokovic as the underdogs in the semis of a big tournament.