The visitors were on the back foot almost immediately after Italian maestro Alessandro Del Piero's sublime individual goal in the seventh minute.

The defending champions bounced back with a Besart Berisha penalty right on half-time and had their chances to hit the front in a highly competitive second half.

But Sydney young gun Peter Triantis hammered home the winner in the 70th minute to lift the Sky Blues into sixth spot and push Roar down to eighth.

"It's amazing the table at the moment, you've got a couple of points from fifth down to 10th and what that lends itself to is a few nerves and we helped Sydney get over their nerves pretty early by gifting them a goal in the seventh minute, and it took us a long time to get into the game," Mulvey said.

"For us, particularly in the first half we didn't have our shape right, our structure wasn't right playing out from the back and that put us under pressure trying to build our play which is what our game is based on.

"We addressed that in the second half and I thought we did better in the second half at that and we started probing.

"We were back in the game and 1-all and we had our chances in the second half with a couple of close misses but low and behold they go up the other end and score a goal so it was disappointing not to get any points today."

Mulvey concedes the brilliance of Del Piero was ultimately the difference between the two teams.

"You've got one world-class player out there who doesn't suffer nerves," he said of the 38-year-old.

"Both teams were coughing up the ball and he's a class player and he makes the difference, he makes things happen.

"He's truly a lovely player to watch if you're a spectator, not if you're an opposing coach."

Roar's cause wasn't helped when Shane Stefanutto had to be replaced at half-time with a stomach bug.

"That disrupted things and it also disrupted us not just when he came off, it disrupted us when he was in the game because a big part of our game is him getting forward on the left-hand side and he was unable to do that," Mulvey said.

Mulvey's charges travel to Thailand on Monday ahead of Wednesday's AFC Champions League qualifier against Buriram United before returning to host Wellington Phoenix at Suncorp Stadium on Sunday.

"We accept the challenge for what it is and we proudly are going to go to Thailand representing Australia and hopefully all of Australia will be behind us and hoping that we put on a good show," Mulvey said of his side's hectic schedule.

"And we'll certainly be endeavouring to do that. It's going to be a difficult trip but one we'll tackle as professionally as we possibly can."