The Saints will travel to AAMI Stadium next Friday to take on minor premiers Port Adelaide for a place in the grand final after thrashing the Swans 16.11 (107) to 8.8 (56) in wet conditions in front of 50,671 fans at the MCG.
The loss continues the Swans' dreadful finals record in Victoria and at the MCG in particular.
This was the Swans' ninth successive loss in finals matches at the MCG and they have still not won a final in Victoria since 1945, while their last finals victory at the MCG was in 1936.
However the Swans, playing their second successive final in wet conditions after winning in the torrential rain and lightning in Sydney last week, simply ran out of legs and fit players against a St Kilda side determined not to go out of the finals in "straight sets" after a brilliant home and away season.
After being so thoroughly overwhelmed and intimidated in Brisbane last week, Thomas recalled experienced players Stephen Powell and Justin Peckett for the knock-out semi-final against the Swans and the decision proved a masterstroke.
Along with the Saints' oldest player in 33-year-old Robert Harvey and fellow veteran Andrew Thompson, the pair set the standard for the Saints' ferocious attack on the ball in the second quarter when Thomas' team put their stamp on the game after a slow start.
After just one goal in the opening term the Saints booted four goals to two in the second term to go in at half-time with an eight point lead and already the Swans were looking in trouble with key running players Paul Williams (corked thigh) and Ben Mathews (ankle) off the ground injured.
And the Saints put the game beyond doubt with a brilliant opening to the third term and it was Peckett who kick-started it with a fantastic goal on the run. And when Steven Baker and Stephen Milne goaled immediately afterwards, the Saints had a match-winning 26 point lead in the slippery conditions.
The Swans launched one last bid to get back into the match and three quick goals to Jason Ball, Michael O'Loughlin and Jude Bolton - after a 50 metre penalty - saw them get within 11 points at the 26 minute mark.
However any hope the Swans had of breaking their MCG finals drought soon disappeared when Milne goaled again from what appeared a soft holding free against Paul Bevan and then Nick Dal Santo goaled right on the three-quarter-time siren to ensure the Saints went in at the last change with a four goal lead.
It was more than enough against a tiring Swans side and with O'Loughlin also showing signs of injury in the final term they just did not have the firepower to get back into the game.
With Barry Hall well beaten by Max Hudghton and Adam Goodes struggling against Jason Blake - who was a real surprise packet for the Saints and not only performed a great blanketing job on Goodes but also had a hand in several goals - the Swans just could not kick goals.
But at the other end, with the marvelous Harvey continually forcing the ball forward, the Saints had no trouble as 2004 Coleman Medalist Fraser Gehrig booted four last quarter goals to finish the Swans off.
Gehrig finished the game with six goals to take his season's tally to 98 and he will have the chance to bring up his first century of goals in next week's preliminary final.
He was ably supported by Nick Riewoldt, who was a constant menace across centre-half-forward in providing the kind of marking target the Swans lacked so badly at the other, as well as the lively Milne - who booted four goals to totally justify his recall to the side.
And considering that Milne, Powell and Peckett - the three players recalled to the side this week - all starred in the win it could well be argued that the Saints' team selections on Thursday night played a significant factor in the club's first finals victory for seven years.
A delighted Thomas quickly moved on from the win and looked forward with a glint in his eye to next Friday's showdown with Port. "This is definitely the year they have to win respect in relation to their finals performances and at the bare minimum they would be expecting to play in the grand final," he said.
Sydney coach Paul Roos said his side needed everything going right for them if they were to win against St Kilda, but he conceded it didn't turn out that way.
"I don't want to take anything away from St Kilda - they're a very good side - but it does (make it difficult). You just find yourself shuffling names and numbers and you can't do the things you normally would do," Roos said.
St Kilda: 1.4 5.6 10.9 16.11 (107)
Sydney: 2.3 4.4 7.4 8.8 (56)
Goals: St Kilda: Gehrig 6, Milne 4, Baker, Ball, Dal Santo, Koschitzke, Peckett, Riewoldt
Sydney: Hall 2, Ball, J.Bolton, Buchanan, Nicks, O'Keefe, O'Loughlin
Best: St Kilda: Riewoldt, Gehrig, Jones, Harvey, Dal Santo, Thompson, Powell, Peckett, Ball, Blake, Hayes
Sydney: J.Bolton, Maxfield, Kirk, Buchanan, Ball, Crouch
Injuries: St Kilda:
Sydney: Mathews (ankle), O'Loughlin (corked thigh), Williams (corked thigh)
Changes: St Kilda: Black replaced in selected side by Guerra
Sydney: Nil
Reports:
Umpires: Allan, McLaren, McInerney
Crowd: 50,671 at MCG
