Whakarewarewa's Te Rangi Fraser judged the wind beautifully to draw the conversion through from outside the right hand upright to make it 7-0 midway through the first 40.
Neither team could score additional points before the break, setting up a tight finish between two sides hearing mixed reports from Greerton Marist's match against Tauranga Sports.
The Mount had much the better of the second half but had just a Cayden Matahaere penalty to show for all their endeavour.
Last season's beaten finalists turned down numerous penalty shots at goal in lieu of using their lineout down the stretch, but found an opposition more than willing to tackle and dig in close to their line. The result, which will see Mount Maunganui travel to play Te Puke and Whakarewarewa away to Tauranga Sports in the semifinals, should see both sides ready for knockout footy this weekend.
Te Puna finished their season in style by running in nine tries against Rotoiti, while Rangataua beat Opotiki 21-17.