The Saints led by a game-high 17 points midway through the final term before a late rally from the Swans saw youngster Gary Rohan level the scores with just over five minutes left on the clock.
A David Armitage behind two minutes later was enough for the win as St Kilda scrapped hard in the dying minutes to get over the line, 2.9.7 (79) to 0.12.6 (78).
The narrow margin made Mark Seaby's third-quarter error - when his decision to play on after a 50m penalty made his goal worth six points and not nine - a costly one.
Swans' coach Paul Roos, however, was able to see the lighter side of the blunder after the game.
"I joked with him at three-quarter time. I said 'if we lose this game by [less than] three points you're going to have to pay the prize money out of your own pocket'," Roos said.
"The boys were giving it to him after the game."
The error was a rare blight on the former Eagle's performance which drew the praise of his coach.
"He played very well … it was his second game at the club and he had 15 possessions and kicked a goal. At this early stage he's a really good pick-up for us - absolutely," he said.
"It's just one of the anomalies of the [NAB Cup] competition because, if you play in a regular season game, clearly you want your player - if there's no one on the mark - you're going to want him to play on."
The Saints had more of the ball early, but their inaccuracy in front of goal allowed the Swans, who were a lot more efficient going inside 50, to lead by four points at the first break.
Two goals in a minute to the visitors stretched that advantage early in the second, but some quick thinking from Brendon Goddard saw him slam through the only nine-pointer of the half to close the gap to one point.
It was the last St Kilda score of the half, however, as Paul Bevan added his second major to send the Swans into half-time with a nine-point lead.
The Saints came out with renewed vigour after the break and looked poised to run away with the contest. Goals to Jason Gram and Justin Koschitzke put their side back in front and a follow-up nine-pointer from Gram had the Swans on the back foot.
St Kilda took a five-point lead into the last quarter with Milne's three goals appearing to do enough for the win before the Swans rattled home with four of the last five goals of the match to set up the thrilling finale.
Saints coach Ross Lyon was unhappy with his side's efforts around the stoppages and their defensive actions around the ground in the first half. His charges were able to move the ball with greater fluency in the second, leaving him satisfied overall with the hitout.
"You wouldn't want to play against any other team in the pre-season if you want a contest," Lyon said.
"They play a hard one-on-one brand, there's no time or space and you've got to work hard for your possessions so really it’s worth its weight in gold in regards to preparation for us.
"We walk away with some improvement, but still with a fair bit of work to do to get back to our best. I thought it was a game of contrasting halves for us in how we went about it.
“I thought it was just a contest to half-time and then I thought both teams lifted their intensity and there was pressure and more tackling in the second half.
"It looked to me more like the games I'm used to our boys being involved in."
St Kilda will play off against the winner of Sunday's match between North Melbourne and Fremantle for a berth in the grand final.
St Kilda 0.2.4 1.3.4 2.5.5 2.9.7 (79)
Sydney Swans 0.3.2 0.6.4 0.7.6 0.12.6 (78)
GOALS
St Kilda: Nine-point goals: Goddard, Gram Goals: Milne 4, Koschitzke, McEvoy, Riewoldt, Gram, Stanley
Sydney Swans: Nine-point goals: Nil Goals: Bevan 3, Jack 2, Seaby, McGlynn, Currie, Moore, O'Keeffe, White, Rohan
BEST
St Kilda: Goddard, Montagna, Gwilt, Milne, Jones, Dal Santo, Riewoldt.
Sydney Swans: Grundy, Mattner, Seaby, O'Dwyer, Kirk, Bevan, Richards.
INJURIES
St Kilda: Nil
Sydney Swans: Nil
Reports: Nil
Umpires: Rosebury, Meredith, Sully
Official crowd: 10,998 at Etihad Stadium
The views in this article are those of the author and not necessarily those of the clubs or the AFL.
