Two days after being upset by NSW, Western Australia looked set for victory at 3-133 in pursuit of South Australia's 166 at Sydney's Cricket Central.
But the three-time defending champions fell apart miserably, with Nathan McAndrew claiming 5-40 as Western Australia were all out for 165 in the 38th over.
The loss leaves Western Australia with their worst start to a season since 2015, and the first time they have suffered back-to-back 50-over defeats in nine years.
The defeat came despite the best efforts of quick Bryce Jackson, who claimed 6-31 and the best figures for Western Australia in the competition's history.Â
Jackson has long been touted as a potential talent in Western Australia, but a series of major back injuries denied him a debut until age 24 last summer.
The right-arm quick took 3-67 in his maiden match against Tasmania in February, before taking his chance on Tuesday with Jhye Richardson rested.
He had Henry Hunt caught behind with a ball that swung away in his first over, before getting Mackenzie Harvey and Daniel Drew in his next over.
And after Liam Scott (46) and Daniel Drew (49) steadied the ship for the Redbacks, Jackson returned to get rid of both of them before beating McAndrew for pace on the hook shot.
While Western Australia wobbled early in their chase, the competition heavyweights appeared to have the match under their control.
With Ashton Turner going after Henry Thornton by cutting him for six and taking 16 off another over, a bonus-point win looked the most likely scenario.
But the match swung viciously after Sam Whiteman pulled Wes Agar down the throat of Thomas Kelly at deep square leg on 48.
Ashton Agar and Baxter Holt followed in similar fashion to McAndrew as the West Australians lost all control of the match.
And when McAndrew had Matthew Kelly caught behind and clean bowled AJ Tye for a duck in his next over, Western Australia still required 10 for victory.
Ben Manenti then landed the killer blow when he nicked off Turner on 53, leaving Lance Morris and Jackson six to get for the final wicket.
Jackson was ultimately stumped trying to drive Manenti, denying him a dream day and handing South Australia their first win over Western Australia since 2017.
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