A late George Blackwood header earned Adelaide United a precious 1-0 win against arch foes Melbourne Victory in their Good Friday A-League fixture.
Blackwood's goal from a clean close-range connection ensures Adelaide will keep fourth spot on the ladder entering next week's last round before the finals.
Victory will finish third regardless of next week's results after losing any hope of pinching second spot from Sydney FC.
On the cusp of the finals, Victory also lost key playmaker James Troisi to a hamstring injury.
Troisi hobbled off Adelaide's Coopers Stadium after 30 minutes, receiving treatment on his left hamstring.
Adelaide move to 41 points, gaining valuable breathing space from Wellington (fifth, 37 points) and Melbourne City (sixth, also 37), who meet on Sunday in New Zealand.
"It was an important result, a big win and a fantastic game from our side," Adelaide coach Marco Kurz said. "Defensively we were very, very good -- very strong. And the boys believed until the end."
Before a bumper 12,866-strong crowd, the Reds controlled much of the territory against Victory but had to wait until the 81st minute to gain reward.
Blackwood was the home-town hero, neatly latching onto a speculative cross into the box from almost 30 metres out by Adelaide captain Isaias.
The goal came from one of few genuine chances in a hot contest although, moments earlier, Victory believed they had broken the scoring stalemate first.
From a 78th-minute Keisuke Honda corner, Melbourne's James Donachie headed into the net only for teammate Georg Niedermeier to be penalised a split second earlier for contact which felled an opponent in the box.
But Victory boss Muscat labelled the contentious call as "incorrect," saying two Adelaide had players collided.
"It should have been given a goal, it was that clear and that obvious," Muscat said. "Would the ... goal, on a night where there were very, very few chances, have made a difference? I suppose that's for everyone to assume.
"There was zero contact or minimal contact from all the replays I have seen ... and then the actual decision, I think, was incorrect."
Adelaide and Victory both finish their regular season on the road. United meet Brisbane on Anzac Day, while Victory play Western Sydney on Saturday night.