The Sky Blues remain undefeated this season, after securing their 12th win, to go with four draws, of the current campaign.
Brosque broke the deadlock in the 58th minute with a wonderful volley that rounded off some solid build-up play. The strike, his fifth goal in his last five A-League matches.
Just three minutes later Sydney doubled their lead, after Adelaide failed to clear the ball, it fell to Ninkovic who made no mistake placing it into the top corner.
While Sydney, at times, had trouble breaking down Adelaide, the Sky Blues always looked in control with the Reds offering very little going forward.
Earlier in the match, Rhyan Grant was perhaps lucky to stay on the pitch after a high-foot tackle on Marcelo Carrusca.
Sydney fans also had right to be frustrated with the referee after Filip Holosko's first-half goal appeared to be incorrectly ruled-out for offside.
Sydney remain top of the table on 40 points, eight in front of Melbourne Victory in second who have a game in hand. Meanwhile Adelaide stays in ninth, equal on 10 points with Central Coast Mariners.