Perth Glory have capitalised on a missed offside call to consign the struggling Sydney FC to a third straight loss this season at nib Stadium on Saturday.
The Sky Blues dominated the match, but it was the Glory who walked away with a 1-0 win and a second home triumph from as many games.
The hosts' goal came when Japanese winger Ryo Nagai was brought down in the middle of the penalty area by Sydney goalkeeper Vedran Janjetovic.
But replays showed Nagai was offside by at least a metre when team-mate Jamie Maclaren crossed the ball.
Referee Ben Williams was quick to blow his whistle and hand Janjetovic a yellow card, with Nicky Carle also booked for remonstrating as the Sky Blues ferociously protested the decision.
Steven McGarry then stepped up to the spot and converted the penalty to put the Glory ahead after just nine minutes.
It wasn't the only offside call in the first half that left players and spectators scratching their heads.
Perhaps perturbed by the initial controversy, the assistant referee called Maclaren offside in the 23rd minute, despite appearing to be onside, just as the Glory striker broke free for a one-on-one opportunity against the Sydney goalkeeper.
The controversy overshadowed the returns of Alessandro Del Piero and Shane Smeltz, who both came off the bench in the second half but had little impact.
Del Piero received a mixed welcome from the Perth crowd when he was introduced in the 63rd minute after missing last week's loss to Western Sydney with a calf injury.
The 38-year-old Italian superstar rejected the offer of the captain's band from former Socceroo Brett Emerton, who worked tirelessly for his side in the first half.
Sydney squandered several golden chances to put the ball in the back of the net, with Carle and Mark Warren both forcing Glory custodian Danny Vukovic into action and Ali Abbas mistiming a header into an open goal.
A spiteful first 45 minutes saw four players booked, including Perth pair Scott Jamieson and Matt Davies for reckless challenges.
Sydney continued to attack hard in the second half, with Richard Garcia's header in the 56th minute just sailing over the bar.
McGarry then looked set to score his second goal of the match after pouncing on a clever pass from Ryan Edwards and running circles around Sydney defender Tiago Calvano, but his shot was narrowly wide of the target.
Matt Thompson had Sydney's best chance of the second half, with his low shot on goal being deflected by Vukovic into the left post and out for a corner.
New Zealand international Smeltz, playing his first match since April following hip surgery, was lively in the final minutes with two attacking runs, but also missed a chance to pass to McGarry in open space.
Perth Glory 1 (McGarry 9 pen)
Crowd: 10,613 at nib Stadium