St Kilda reigned supreme in a sodden AFL semi-final, thrashing Sydney by 51 points at the MCG.

Stepping stylishly out into their first game in the wet this season, the Saints soaked up Sydney's pressure as the rain fell early then they surged ahead through six successive goals.

The 16.11 (107) to 8.8 (56) win before a crowd of 50,671 earned St Kilda a preliminary final against Port Adelaide at AAMI Stadium next Friday.

While the rain eased off in the second half, the Saints played the best football in the conditions by moving the ball by foot, handling it little and overwhelming Sydney with fierce defensive pressure.

Coleman medallist Fraser Gehrig was the beneficiary of the slick movement and booted six goals to take his season tally to 98, while Stephen Milne proved an inspired selection with four goals from a forward pocket.

St Kilda captain Lenny Hayes was outstanding with 12 tackles, Aussie Jones vindicated his All-Australian selection across half-back and veterans Robert Harvey - in game No.299 - and Justin Peckett were tireless throughout.

The Swans finished the night both out of puff and out of personnel.

Heavy legs from last week's win over West Coast in the wet were not helped by injuries to Paul Williams, Ben Mathews and Michael O'Loughlin.

Sydney looked aided by the rain early, and kept the Saints to one goal in the first quarter.

But the Swans lost their way in the second term as St Kilda kicked the last three goals of the first half to take the momentum into the main break.

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