After scoring 5-366, the hosts continued their hot form by skittling the Port Boys for just 143 to claim first-innings points.
Cameron Huckett gutted Fremantle’s middle order to finish with figures of 4-19 from 13 overs.
His impressive work followed that of Alex Bevilaqua and Sam Birchall, who set their side on the right path by removing Fremantle’s top three batsmen cheaply.
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READ NOWHuckett kept the momentum in South Perth’s favour to leave Fremantle all but out of the contest at 5-36.
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Ben Sutton (28 from 74) and Chris Chellew (31 from 81) provided Fremantle’s only real threat to the scoreboard, while Jarryd McPhee dug in with a 68-ball 14.
Fremantle will need to rebound quickly, taking on Perth at Stevens Reserve on Saturday, and the one-day grand final against Claremont-Nedlands on Sunday.