In reply to South Australia's 9-208, WA were dismissed for 164 in 38.5 overs in Thursday's match at the WACA Ground.
The result catapulted South Australia above NSW into top spot, putting them in prime position to host the March 1 final with just one round remaining.
WA, winner of the past three titles, sit last with just one win to their name.
South Australia's innings featured several handy contributors but no massive scores.
Liam Scott top scored with 47, while Jason Sangha (38), Nathan McSweeney (28), Mackenzie Harvey (24) and Henry Thornton (23) all made solid starts.
WA paceman Mahli Beardman's growing reputation was further enhanced after he ripped through South Australia's middle order in taking 4-46.
Beardman bowled Daniel Drew for a duck in the eighth over, and the 19-year-old knocked over the castles of Harry Matthias (14) and Scott later in the innings.
He claimed his fourth scalp in the penultimate over when Henry Thornton was caught in the deep.
Spinner Corey Rocchiccioli (2-38 off 10 overs) enjoyed a solid debut, while tearaway quick Lance Morris (2-34) set the tone with the wickets of Henry Hunt (15) and Sangha.
WA unleashed three debutants in Thursday's One Day Cup clash - Jayden Goodwin, Joel Curtis and Rocchiccioli.
It meant they fielded one of the most inexperienced outfits in recent memory, with the trio of debutants joining Beardman (second game), Keaton Critchell (third game), Sam Fanning (third game) and Bryce Jackson (fourth game).
WA were cruising at 0-40 in the sixth over before a superb catch at first slip by Sangha changed the match in a blink of an eye.
Sangha dived to his left and was horizontal when he snared the spectacular catch to dismiss Fanning (34 off 19 balls).
It sparked a collapse of 4-5 as Goodwin (six), Hilton Cartwright (0) and Curtis (2) departed in quick succession.
WA were humiliated by six wickets inside five sessions during their recent Sheffield Shield match against South Australia.
The Warriors were skittled for 120 and 66 in that clash, and their collapse on Thursday threatened to produce another embarrassment.
A 77-run partnership between D'Arcy Short (55 off 68) and Ashton Turner (43 off 40) catapulted WA back into the contest, but their departure sparked a collapse of 6-42 as South Australia cruised to victory.
Redbacks allrounder Scott finished with 3-16 from eight overs, while Wes Agar, Lloyd Pope and Harry Conway snared two apiece.
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