Sydney blew their chances and Isaac Heeney might have blown his Brownlow Medal eligibility as St Kilda upset the AFL ladder leaders by two points.
As the Saints draw on Melbourne Storm's example to prepare for long-term success, they rallied from five goals down in the third term to beat the Swans 13.6 (84) to 11.16 (82) on Sunday at Marvel Stadium.
It came after a tough few days for the Saints, who lost key forward Max King for the rest of the season with a knee injury.
The Saints were outstanding, kicking four goals to none in the last term and holding their nerve.
Recalled defender Dougal Howard sparked their rally midway through the third term, ending up in the crowd as he desperately lunged to touch a certain Sydney goal on the line.
St Kilda coach Ross Lyon revealed post-match that personnel from the Storm had visited the Saints last week to talk about the NRL team's sustained success since their salary-cap punishment.
After making the finals last year, the Saints are 6-10, and even with Sunday's big upset the top eight looks beyond them this season.
Lyon said the relationship with the Storm is "a blessing".
"We can't get stuck in win-loss, it's about training, (preparation), standards and setting a foundation for future success," he said.
"We just know it's unlikely, and probably impossible, to play finals - but how we go about it now, train and prepare and compete and review, will set standards for future success."
By contrast, Sydney's impressive season has hit a speed bump, losing narrowly for the second straight week despite more scoring shots.
Also for the second week in a row, key forward Logan McDonald missed a crucial shot on goal late in the game.
Last week it would have given them victory over Fremantle, this time he would have put the Swans in front with less than two minutes left.
"Conversion, as Leigh Matthews said, is probably the most important stat in AFL footy," Sydney coach John Longmire said.
"You can't say much (to McDonald), those things happen in footy. That's part of growing up ... experience teaches you a lot."
Longmire noted Sydney had 54 to 41 inside-50s, but sprayed 3.11 to 9.2 in the second half and also lost the tackle count 63-40.
"We've had a pretty reasonable season. You're going to go through a patch - it's hard to play at that level for a long time," he said.
"It will give us something to work on."
Heeney is in trouble after he fended off opponent Jimmy Webster and caught him high, leaving the Saints defender with a sore nose.
It is certain to come under match-review scrutiny and could mean a suspension that would make Heeney ineligible for the Brownlow.
But the Swans are on the verge of regaining veterans Luke Parker and Callum Mills, with Parker back from his VFL suspension this week and Mills ready to resume from his off-season shoulder injury.
"They'll come in at a good time, they'll be important," Longmire said.
Ruckman Rowan Marshall and defenders Jack Sinclair and Callum Wilkie were among St Kilda's best, while young gun Mattaes Phillipou had an awesome final term in the midfield and Liam Henry also impressed.
McDonald kicked two goals in the first term for the Swans, and Errol Gulden racked up 28 disposals for the game.
Saints onballer Marcus Windhager was subbed out of the game with a hamstring injury.
