Djokovic leads their previous meetings 7-2. He has won their last 5. Nishikori won 2 sets in those matches. One was on clay and one indoors.
Djokovic has been very focused in this tournament and has not lost a set. Thiem had a lot of break chances against him but broke just once. In the semi final Goffin pushed him closer than anyone with an excellent performance.
Nishikori has struggled a little with his serve though was better from the service line against Kyrgios. He is the first opponent in this tournament with the experience and the game to trouble Djokovic. I think he will be able to keep one of the sets tight but its hard to see him beating Djokovic on this surface.
No value unless Nishikori takes a break and a hold lead.