I'm expecting him to choke, BUT we already know Ash is going up against Sawyer in the semis and Alan in the finals.
The last time Ash battled Sawyer, he lost - and he's lost to Alan twice. It would be a little weird if they put Ash up against Alan and had him lose a third time, as they usually set up a rival like that so Ash can triumph the third time as a sign of how far along he's come (see Brandon in the Battle Frontier, or Paul in the Sinnoh League). Obviously it'll end with Mega-Charizard X versus Ash-Greninja - or Pikachu.
Ash almost beat Diantha in a one on one match this series remember, so the writers are clearly setting him up as a powerful trainer this round. Let's see if they dare give him a win.
Weird too that they're setting up things like Serena's (unrequited?) crush on him, or Ash-Greninja which seem like they'd have longer term effects. Even if Ash doesn't win the league, I don't think Alola will be the typical "Ash ditches all his friends and Pokemon to start fresh in the new region" reset this series does frequently, but we'll see. Sinnoh had plenty of foreshadowing that Ash might pull it out then and they yanked it out from him at the last second.