The Alcohol.Think Again Western Warriors set the Redbacks an unenviable 525 for victory, with the hosts needing a win in the final-round match to move off the bottom of the ladder.
South Australia was unable to negotiate a tricky five-over spell late on Friday and was 1-7 at stumps, with first innings' top-scorer Conor McInerney dismissed for a duck off the second ball of the innings.
An unbeaten century from middle-order batsman Hilton Cartwright and a sparkling half-century from spinner Ashton Agar allowed the Warriors to pile on the runs, before eventually declaring at 7-370.
Cartwright was unbeaten on 111 at the time, while Agar hit an entertaining 86 off 134 balls, as the home side struggled without a recognised spinner in their ranks.
SA enjoyed a promising start with WA's overnight pair D'Arcy Short (66) and Ashton Turner (41) quickly removed.
In-form left-arm paceman Nick Winter was a positive for SA, with the rookie returning figures of 4-90.