Melbourne Victory's title defence is on shaky ground after slumping to a shock 1-0 loss to struggling Perth Glory at nib Stadium on Wednesday night.
A 74th-minute goal to Glory import Diego Castro stunned the defending champions, who are looking vulnerable in fourth spot following three straight defeats.
And it doesn't get any easier for Kevin Muscat's charges, who face a white-hot Melbourne City outfit in Saturday's crunch derby.
Glory rose to eighth on the back of their second straight win, lifting them to within four points of the top six.
But they were left with two fresh injury concerns after Nebojsa Marinkovic (hamstring) and Marc Warren (foot) suffered setbacks.
Victory striker Besart Berisha was left fuming after being denied a first-half penalty.
Berisha appeared to have been pulled back by a Glory defender in the 42nd minute as he ran into the box in an attempt to get on the end of a pass.
Referee Chris Beath waved on play much to Berisha's disdain, with the 30-year-old Albanian angrily pleading his case to no avail.
Berisha almost put Victory ahead in the 65th-minute, only for his thundering shot to cannon into the crossbar.
Victory struggled to create many clear-cut opportunities in the first half, and they were lucky to survive a few nervy moments at the other end.
Melbourne goalkeeper Danny Vukovic pulled off a sharp save to deny Josh Risdon's low drive in the second minute, and the former Glory custodian dove full length to tip away Gyorgy Sandor's long-range strike.
Seventeen-year-old Glory striker Jamal Reiners showed some nice touches in his first A-League start, while Risdon was a constant menace down the right.
Perth had their own penalty claims waved away two minutes later when Chris Harold was tripped up by the outstretched hands of Vukovic.
Replays showed Vukovic got a slight touch on the ball before making contact with Harold's foot.
Vukovic saved the day on that occasion, but there was little he could do when Reiners made a neat run into the box before squaring to Castro, who had an easy tap-in.
Castro came within centimetres of scoring his second in the 87th minute, only for Vukovic to tip the free kick away.
The win was sweet revenge for Glory, who lost to Victory 2-0 in the FFA Cup final last month.