Djokovic is not the same player he used to be. In my opinion he could have definitely upped his level there in the 2nd set and forced something when Opelka was on a run of 2nd serves, but he didn't go for them.
Opelka ultimately ended up winning more 2nd serves than Djokovic and that tells a story in itself. Now focus turns to Melbourne for Djokovic where he will get much needed advice and expertise from Andy Murray.