A-League: Sydney 2 Brisbane Roar 5
Despite going behind to a brilliant early Alessandro Del Piero free-kick, the Roar had too much class and firepower and set up the easy win with three goals before the break in front of almost 18,000 fans at a wet Allianz Stadium.
Thomas Broich, playing in a central striking role in place of the injured Besart Berisha, got the equaliser before Petratos came back to haunt his former club.
The 21-year-old, who had a messy split with Sydney last year before leaving for Malaysia, converted a penalty in each half and also finished off a sublime team move to complete his first senior treble before Ivan Franjic and the hosts' Richard Garcia both scored in stoppage time.
The victory sees the Roar become the first side in the competition to notch 100 A-League wins and ensures Mike Mulvey's side head into 2014 on top of the table.
Sydney have now lost three of their last four games and their horror show was compounded when Del Piero was forced off at half-time through injury.