Chicago (7-15-6) won for the second time in its last 14 matches. Orlando (7-18-3) has only eight points since May 1.
In the opening minutes, Dax McCarty stole a back-heel pass and played it ahead to Michael de Leeuw, who rolled it across goal for Nikolic's redirection. In the 70th minute, Raheem Edwards intercepted a pass near midfield, dribbled it to the 18-yard box and found a trailing Nikolic for his 14th goal of the season.
Aleksandar Katai made it 2-0 in the 28th. Goalkeeper Joe Bendik had a poor first touch on a back pass and Djordje Mihailovic rolled it across an empty net for Katai's easy finish. De Leeuw, who recently returned from an ACL injury last season, tapped home a redirected corner kick in the 56th.
Chicago goalkeeper Richard Sanchez denied Dominic Dwyer's breakaway shot in the 64th.