A loss to Melville snapped Rockingham-Mandurah's five-game win streak in the first week of finals action over the weekend, with the minor premiers now forced to use the second chance they garnered from finishing atop the Western Australian Premier Cricket ladder.
The Mariners won the toss and batted first against the Storm and found the going tough against a disciplined bowling attack.
Tyler Robertson was the glue that held the top order together with a 160-ball innings that lasted three-and-a-half hours.
When the opener was dismissed for 50, the hosts had lost half their batsmen with 93 on the scoreboard.
Brandon Jacobs and Brandon Burt combined for a sixth wicket stand of 76 that gave hope of a total to defend.
Jacobs made his maiden half-century a timely one, striking eight boundaries in a pugnacious knock of 54.
Burt's 48 not out signaled the selectors' decision to include him for his first game of the season to replace James Bracey was a prudent one to add some middle order punch.
Rockingham-Mandurah struck on the last over of day one through Craig Simmons, leaving Melville at 1/0 in pursuit of 200.
The Storm's Fraser Hay (87) and Sean Terry (61) took the challenge up on day two despite the ever-present spin threat of Simmons (3/66), to ease to 3/202 in the 57th over.
Rockingham-Mandurah clash with Fremantle on Saturday and Sunday from 11am at Lark Hill for a place in the Grand Final at the WACA, while Melville meet Claremont-Nedlands for the other berth.
