Part of that could be nerves.
Mental pressure can make feel quite tired. He isnt the fav in the final. Whereas today he was even on to make the final.
He is 26 and has two days rest. No reason he shouldn't be rested and ready.
I dont think fatigue will come in here. Dom is young and seems very fit and Novak is a different level that most.
I imagine Novak will be looking to wear him down early on. Just want to see a good match and hopefully 5 sets!