Already understrength, the Swans rallied to a 21.15 (141) to 13.10 (88) triumph after captain Callum Mills went down with a knee injury in the opening quarter on Saturday at the SCG.
Logan McDonald brought the energy against a lacklustre Lions outfit with a career-best six-goal haul, while Charlie Curnow and Tom Papley kicked five majors each in a near-complete Swans performance in front of 37,261 fans.
Bracing for scans to confirm the dreaded ACL tear for Mills, Sydney will host either Fremantle or Geelong in a preliminary final as they push for an improbable premiership.
The Swans are already without vice-captain Isaac Heeney, Chad Warner and Nick Blakey, who are among five players serving club-imposed suspensions.
Brisbane will return to the Gabba and host either the Western Bulldogs or Adelaide in a semi-final, as they continue their pursuit of a third consecutive flag.
Many critics had ruled second-ranked Sydney out of flag contention, and even club chairman Andrew Pridham said the Swans' chances had been "torpedoed" in the fallout from events at East Melbourne's Pullman Hotel last month.
Asked if the commentary around Sydney's premiership push had added to motivation, Cox said: "Part of it was, yep.
"Things we'll talk about within our four walls are things that will stay there, and we've still got a huge job to do.
"It was a real sign of a galvanised group who were hungry to show that they're capable."
Brisbane coach Chris Fagan had warned his side against complacency, saying his message was "respond".
"You talk about, 'Did you really think that Sydney were going to be that good, or did you think it was going to be maybe a walk in the park?'," Fagan said.
"'Because that's how it had been painted by all and sundry that's following football, you know?
"And probably quite rightly so, but you can never underestimate the human spirit."
Cox couldn't provide an update on Mills, only that they were expecting the worst.
"We're not sure exactly the full extent of it, but it doesn't look good," the Swans coach said.
McDonald was excellent against star Lions defender Harris Andrews, finding redemption in September against the Lions.
The 23-year-old had been kept to one disposal in the 2024 grand final loss before missing the entire 2025 campaign through injury.
Papley had shaken off a left calf issue - picked up during warm-up - to pull off an equal career-best haul, leading alongside midfielder Errol Gulden (34 disposals, one goal) and Hayden McLean (three goals).
Swans midfielder James Rowbottom was also superb in nullifying Lachie Neale (18 disposals).
Oscar Allen kicked four goals for Brisbane, while Charlie Cameron had one behind from four disposals.
Brisbane co-captain Hugh McCluggage (16 disposals) was still rusty on return from a calf injury, leaving it to veteran defender Dayne Zorko (26) to lead the way for the Lions.
Sydney won every quarter in a near-complete performance, setting the tone by firing off six goals to two in a blistering start.
Brisbane got on the front foot after the first break when Allen kicked back-to-back majors, but the Lions couldn't find the spark for a comeback
Inspired by livewire teammate Papley, Gulden finally hit the scoreboard in the third quarter to send the SCG crowd rocking to Australian Crawl's 1981 hit "Errol" and break the 100-point ceiling.
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