Sydney put a foot through the AFL finals door and probably slammed it shut on St Kilda with a 51-point victory at the SCG on Saturday night.

The Saints started well, but were out-gunned in the second half with Sydney kicking nine goals to three to win 15.22 (112) to 9.7 (61) in front of 27,558 fans.

In the third quarter, Sydney had ten scoring shots to two in the third quarter to lead by 22 at the last change and added five goals to one in the final term.

It was Sydney's ninth victory from its last 11 starts, while the Saints slipped to their fourth successive loss in what coach Grant Thomas had labelled a must-win game.

The visitors led 15-2 early on and played with plenty of verve against a sluggish-looking Swans outfit.

While Sydney turned a two-point quarter time deficit into a four-point halftime lead, it was only in the second half the home side displayed the dashing play which has characterised its surge up the ladder.

With big men Jason Ball and Adam Goodes prominent, Sydney won a constant stream of good possession out of the centre and converted on several occasions, even though it ended up with a minor score on numerous occasions.

Sydney-born Saints star Lenny Hayes was held to just 11 touches by in-form Tagger Brett Kirk.

While former Saint Barry Hall went goalless, Sydney spread its goalkicking load among ten other players with utility Matthew Nicks and mid-fielder Daryn Cresswell each slotting three and Ryan O'Keefe two.

Saints showed little potency up forward where skipper Aaron Hamill was nullified by Craig Bolton.

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