The Mounties had a bye in their final pool round.
On day one Tauranga Sports lost to Nelson Wanderers 24-10 and Auckland's Suburbs 31-17 before beating Old Boys University 21-10 to make the Bowl quarters, where they had a bye through to the showdown with Mount Maunganui.
Rangataua, the 2014 national champions and Middlesex Cup winners, were the best of the Tauranga teams in making the Cup quarter-finals.
They began with a 17-12 opening win against Wellington's Poneke and then shocked the home crowd by defeating defending champions Wainuiomata 15-12.
Rangataua trailed 12-5 at halftime and defended courageously to hold the rampant home side scoreless in the second spell.
After Rangataua closed the gap to 12-10, Wainuiomata bombed two try opportunities, knocking on over the line and then being penalised for a double movement in the final seconds.
Rangataua took full advantage to take a quick tap and run the length of the fieldfor the matchwinner.
Eventual champions Melville from Hamilton then beat Rangataua 22-0 but the Maungatapu-based club still made the Cup quarters where they lost to Counties Manukau side Karaka 15-12 in a close-run thing yesterday morning.