PSG host relegation battling Le Havre on Saturday
The world number one was far from his imperious best and struggled on serve but was content to allow his Japanese opponent to fall on his own sword with 54 unforced errors under the Rod Laver Arena floodlights.
Djokovic fired a sizzling cross-court backhand winner to close out the match and will make his sixth appearance in the last four at Melbourne Park, having avenged his loss to Nishikori in the 2014 US Open semi-finals.