Fullback Zyon Hekenui coolly landed three straight penalties to give Ruapehu a nine-point advantage they didn't look like relinquishing.
But, finally galvanised, as Pirates' desperation increased so did their passing improve, with first five Denning Tyrell lifting from a middling first half to begin to dictate the terms.
No8 Lasa Ulukuta upped his tempo with some powerful runs, while prop Brett Turner gutted out a broken finger in the fourth quarter and was inspiring with his great offloads, aggressive mauling, and smart thinking to have a quick halfway lineout when Ruapehu kicked off on the full.
Closing the gap through a Tyrell penalty, Pirates finally broke Ruapehu in the 66th minute as a kick charge-down was spread to winger Dawai Navitio to score in the far corner, with Tyrell raising the flags from the sideline.
Pirates cleared their own territory with some big runs by winger Clive Stowers, while Ruapehu were perhaps guilty of staying too much in their element of kick-and-chase, handing back possession when they needed it most as Etuati and Tyrell held firm.
Just returned Wanganui representative midfielder Pehira Huwyler came off the bench to make some big charges, but found Ulukuta and the Pirates pack up to the task.
Facing the prospect of potentially two more match-ups with Ruapehu at Spriggens Park - Round 2 and the final - Pirates coach Phillip Morris wanted to keep the big win in perspective.
"You could say it's an early-season game.
"I thought our fitness was up there and our set pieces improved."
And while it was his first loss as a coach to the city club, Ruapehu's Chris Winter knew the two sides were more evenly matched than his team's previous winning record suggested.
Nor would he fault his playmakers for persisting with attacking kicks late in the game.
"They had the thought of trying to stay in [Pirates] half - it's about building pressure and then building off that. I've got no complaints.
"It was a game where it could have swung either way."
After absorbing early pressure, Pirates struck in their first venture into Ruapehu territory as Etuati got outside his markers after a smart passing chain and cut back in to go under the posts.
Ruapehu immediately replied as the tacklers fell off Vaughan, skipping through to feed Hekenui who put Roberts over untouched, the young fullback then converting from the sideline.
Pirates' missing touch set up another big run by Roberts and opened up a penalty attempt for Hekenui, who then swiftly added another as Pirates got trapped in their own 30m zone.
He had a third six minutes before halftime as Ruapehu looked set to either sit or build on their advantage.
But Pirates found their rhythm as Turner, Stowers, lock Simon Hillis and centre Kameli Kuruyabaki initiated some deep raids, which came unstuck when the final pass went down.
Winter screamed at his team from the sideline to "keep the energy", but it was Pirates who now made all the running.
After Navitio's try and Tyrell's excellent conversion, Pirates absorbed Ruapehu's desperate finish, with Tyrell willing to try short chip kicks to keep them at bay, while Ulukuta snuffed out Huwyler's last ditch raid with a big hit on fulltime.
