Brisbane Roar's ruthless gutting of a dysfunctional Sydney FC outfit at Allianz Stadium on Boxing Day was so emphatic it's hard to know how the Sky Blues will recover.

This was death by possession, capped with the menace of shock and awe in attack and topped by a hat-trick to ex-Sydney forward Dimi Petratos.

The Sky Blues scored first through - who else - Alessandro Del Piero, but were subsequently picked apart by a team playing at a different class.

Now is probably an inopportune time for Sydney fans to be reminded that Brisbane coach Mike Mulvey, the brains by the Roar's stunning revival, was in the final three for the vacant Sydney coaching job when Ian Crook resigned.

One had to feel for Frank Farina, watching his old team take a machete to his team for the second time in two months. A similar massacre took place - the scoreline was 4-0 - at Suncorp Stadium.

By half-time, the tale of the tape was utterly grim. The Sky Blues had blown their lead and conceded three times, each goal more howling than the last.

Del Piero was to blame for the equaliser, refusing to get back onside when the ball came to Thomas Broich, and the Roar's marquee man prodded home. Barely a few minutes later, Henrique worked his way around Sebastian Ryall and tore through on goal, only for Ryall to haul the Roar forward down. Petratos, once seen as Sydney's next big thing, made his old club pay from the spot.

It was a sweet goal for Petratos, not least given his acrimonious departure from his home city a year ago when he suffered a dramatic falling out with assistant coach Zeljko Kalac.

But there was more glory to come for the 21-year-old as he had the final touch on what may well be voted the goal of the season.

A fluid breakaway by the visitors from the last line of defence saw most of the team touch the ball before Broich fed Petratos, searing down the right, to belt a swerving shot past Vedran Janjetovic. Not that the keeper was to blame for any of the goals - one save on the brink of half-time from Petratos was from the top draw.

Theoretically, a two-goal margin meant Sydney still had the hope of a point, but the 17,662 on hand knew better. Plenty fled for cover as the rain began to fall; just as many headed for the car park.

It was at this time that Del Piero, as is becoming all too common, took his place on the bench for the second half.

However, his first-half free-kick to open the scoring against the run of play was a thing of beauty. He calmly dipped the ball inside the right post, and while Michael Theo saw it coming and made contact, he couldn't keep it out.

That was as good as things got for Sydney all night. This was a Christmas tease.

Sydney's midfield was slaughtered thereafter by the A-League's most mobile unit. Luke Brattan, Matt McKay, Liam Miller and Broich passed the Sky Blues to death and created many chances. The resistance was feeble at best.

In the second half, Brisbane relaxed but still looked far and away the better team. Petratos was denied by Janjetovic again, Matt McKay hit the post and substitute Kwame Yeboah - in his last game for the Roar before joining German club Borussia Monchengladbach - blasted wide from close range.

However, a final sting came when Nikola Petkovic clumsily knocked down Broich on the edge of the box.

Petratos stepped up and made no mistake from the spot for his hat-trick. When Ivan Franjic broke clear in injury time, he showed no mercy to make it five of the best.

Only a Roar mix-up at the end allowed Richard Garcia to ease the humiliation.