St Kilda are through to the semi-finals of the AFL pre-season competition after a blunder by Sydney ruckman Mark Seaby proved very costly at Etihad Stadium on Saturday night.

Seaby had the chance to dob a nine-point goal in the third quarter but had to settle for six points when he played on before kicking, which proved decisive in St Kilda's one-point win 2.9.7 (79) to 0.12.6 (78).

The Saints two nine-point goals were both kicked from just beyond the 50-metre line.

They looked to have the game sewn up several times in the last quarter, but the Swans mounted several late charges and levelled with about five minutes left when youngster Gary Rohan kicked a goal.

St Kilda's David Armitage then earned his side the lead when he booted a behind tucked up close to the boundary line.

Goalsneak Stephen Milne kicked four goals for the Saints, including three in the last quarter, while Jason Gram, Lenny Hayes and Nick Dal Santo all enjoyed successful first outings for the year.

Paul Bevan kicked three goals for the Swans and youngster Kieran Jack two, but it was Seaby's mistake which proved the difference.

The former West Coast Eagle had rucked well for his new side when he earned a free kick and was awarded a 50-metre penalty midway through the third quarter, and trotted to 15 metres out.

Because there was no St Kilda player on the mark, Seaby ran on and kicked a goal, but was deemed to have played on, which cancelled the nine-point reward.

The goal stood, worth just six points.

Gram and Brendon Goddard highlighted the impact nine-point goals can have when they landed long bombs.

Goddard's major kept the Saints in the game in the second quarter, while Gram capped a decisive Saints burst at the start of the third term, when they quickly wiped off the half-time deficit through some strong tackling and three goals.

St Kilda will next play the winner of Sunday's clash between North Melbourne and Fremantle, while Sydney will have a practice game next weekend.

Sydney were good in the first half, mainly through the dash of Tadhg Kennelly at halfback, the pace of newcomer Lewis Jetta, Seaby's presence and a busy Adam Goodes at centre half-forward.

But the Saints were the better side in the second half, as Gram became damaging, Leigh Montagna generated plenty of drive and James Gwilt and then Sam Gilbert reduced the impact of Goodes.

Sydney's Ryan O'Keefe got through his first game of the pre-season, while ex-Cat Shane Mumford rucked for the first time for his new club.

Sydney coach Paul Roos said he was not aware a nine-point goal became void when a player played on, and said the rule was an anomaly between rules in the pre-season and premiership season, and held nothing against Seaby.

"I joked with him at three-quarter time, I said 'If we lose this game by three points you're going to have the pay the prizemoney out of your own pocket'," he said.

"There's the irony of it, we lost by a point so the boys were giving it to him after the game.

"But in a regular-season game you're going to want your player to run in ... and kick the goal, so that's the way it is."

Roos was not too fazed by the defeat, as St Kilda "played a little bit better than us anyway".

Saints coach Ross Lyon was pleased with patches, but said his side still had improvement in it.

"It was a game for us of contrasting halves in how we went about it," he said.

"It was just a contest to half-time and both teams lifted their intensity and there was more tackling and more pressure in the second half and it looked to me more like the games I'm used to our boys playing."

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