The Lions chased down the Tigers’ total, producing one of the shocks of the season and giving them a shot at their first flag since 2011.
Winning the toss and bowling, the Lions dismissed the Tigers for 240.
The Subi chase started strongly, putting on 69 for the first wicket.
Alex Malcolm (32) and AJ Murphy (47) set a strong platform before Callum How (36) and Aaron Lilly steered the Subi side towards an unlikely victory.
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READ NOWGerry Fatorous put the finishing touched on the win with 22, with the visitors defying the odds to reach the grand final.
The Tigers, despite losing just one game for the year, exit the competition.
The grand final will take place from 10.30am next Saturday at the WACA Ground.