The scoreline was perhaps harsh on Adelaide United but the Sky Blues fully deserved their victory.
Alex Mullen was signed by Sydney as a "mature aged rookie" and although he hasn't seen any top division action in the A-League, he was pivotal to this win.
Mullen played up front rather than in his more natural midfield position, and he opened the scoring with a well taken finish with only 12 minutes on the board.
Adelaide United goalkeeper Daniel Margush, who made a shock A-League debut a fortnight ago and was on the bench in Brisbane last night, was forced to save a free kick from Max Burgess, before Chris Zuvela doubled Sydney's lead, finishing after a wonderful run by the lively Daniel Araujo.
Jacob Tratt's own goal gave Adelaide United hope, but it was snuffed out before the break when Burgess lashed in a left-footed shot from the edge of the area.
Mullen powered a free header past Margush early in the second half and when Cristian Gonzalez scored directly from a corner before the hour mark, all hope was gone for Adelaide.
Samed Altundag's goal was well taken after a lapse by the Sydney defence, but it was nothing more than a consolation.
Sydney FC 5 (Mullen 12' 49', Zuvela 24', Burgess 40', Gonzales 56')
Adelaide United 2 (Tratt OG 32', Altundag 61')
Bluetongue Stadium, Gosford
Referee: Lachlan Keevers