Australia will face West Indies in an ODI tri-series final after South Africa meekly surrendered to the hosts by 100 runs in Barbados.
Victory in the ninth game of the series came with the carrot of a place in the series decider, which starts at 3am AEST on Monday.
The pressure was too much for the Proteas, who capitulated with both bat and ball at Kensington Oval.
West Indies collapsed to 4-21 after being sent in but managed to post a total of 285 thanks to a fine century from Darren Bravo.
Bravo was on 11 when Wayne Parnell put down a difficult catch at fine leg.
South Africa crashed to 6-65 in response, with their star-studded top order failing to produce an innings of substance.
The tail wagged but the inevitable conclusion was reached after 46 overs as the Proteas enhanced their reputation for folding in knockout fixtures.
West Indies skipper Jason Holder, who was in doubt for the game because of a hamstring injury he suffered bowling against Australia earlier this week, took the new ball and incredibly delivered a 10-over spell.
Holder removed JP Duminy, while Shannon Gabriel claimed the key scalps of Faf du Plessis and AB de Villiers.
In the opening hour of play it looked like Steve Smith's side would almost certainly face the Proteas in the decider.
Kagiso Rabada took control of the contest in an electrifying opening spell of express pace.
Rabada claimed three wickets, yorking in-form veteran Marlon Samuels for a golden duck.
But the Proteas relinquished the upper hand as Bravo (102) and Kieron Pollard (62) shared a 156-run stand.
Bravo struck 12 fours and four sixes off 103 balls in what was his third ODI ton, while he was part of West Indies' highest fifth-wicket partnership in the format.
Holder (40) and Carlos Brathwaite (33) belted a further eight boundaries to pile more misery on the visitors.
Morne Morkel and Imran Tahir put on a final-wicket stand of 51 runs but the result was well beyond doubt when they reached the crease.
Embarrassingly for South Africa the tailenders' effort was comfortably their highest partnership of the game.
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