However, captain Gerrard Hobbs would bat on with the tail, scoring 49 at a run a ball, helped by a good lower order cameo of 17 not out from 11 balls by Ryan Slight, as United worked through to an imposing 197-8.
Set a run rate of 5.47, Tech got a watchful start from Kinnerley with 23 from 42 balls, as United were able to put the pressure on at 49-2 after 13 overs.
They unveiled a surprise in Chris Sharrock, who did not play wicketkeeper but instead trundled some effective medium pacers as he got dangerman Nick Harding cheaply and then Sam Roebuck first ball.
It fell to captain Dominic Lock to hold firm and he responded with 88 not out from 87 balls, taking the team from 76-5 after 19 overs to a hopeful 135-6 after 27, thanks to a good partnership with brother Tyler Lock (31 from 31).
However, the pressure in the field told as Tech was subject to two run-outs, with Dominic Lock the survivor, to have them struggling at 140-7 in the 31st over.
Needing 44 runs from four overs, Lock carried on and was well assisted by young Quinn Mailman, with 17 not out from 16 balls, but United set their field wide and managed to protect the boundary for their spinners.
Twenty-one runs were needed off the last over, which became 18 from three balls as Power managed a dot ball to Lock, at which point United visibly relaxed.
But in a twist of defiant irony, Lock smashed the last two deliveries for six, losing a ball on the first slog, for his team to finish one scoring shot away and wondering what might have been.
"Quinn did well at the end there," Lock said afterwards.
"We just bowled too many wides - they got 18.
"It was 20 runs too many. Having one more player would have been nice."
Hobbs, remember the shock of losing last year, was happy with United's performance, especially their batting on a flat track after the recent run of low team totals.
"We got a pretty good score, got more than I thought we'd get.
"Dom batted well, put us under a bit of pressure."
At Springvale in the 34-over game, Marton were in big trouble at 14-3 as Sam Beard (3-21) got amongst the top order, who fell to catches.
That brought in-form batsman Hamish Harding together with noted pinch hitter Dan Ford, both making 33 at less than a run a ball with seven boundaries and three sixes between them.
Both men hit hard down the ground and at 71-3 Marton were back in business.
However, Ford tried to push at a wide ball and was caught back from the wicket, then Harding thought a certain four could still be a safely run two, yet Johno Quirk's surprising throw back to Beard was right by the stumps, leaving Harding short of his ground.
From 88-5, Marton still got some fight from John Anthony down the order and recovered somewhat as they were bowled out for 163, with Todd Inness also getting three wickets.
Facing a run rate around 4.79 to start, Marist were composed and knocked off the target with two and a half overs to spare.
This was led by the big partnership between Craig Thorpe (74 not out) and Wanganui representative captain Morgan Inness (64 not out) to see them to 164-3.
Marist and United will now meet in the knockout final on December 12, and the winner of that game will face the Taranaki one-day champions on January 24.
Results
United 197-8 (G Hobbs 49, R Power 37, G Smith 35, R Kinnerley 2-20, T Roebuck 2-27, T Czerwonka 2-23) beat Tech 192-7 (D Lock 88no, T Lock 31, R Kinnerley 23, C Sharrock 3-37) by five runs in a 36 over match.
Marton 163 (H Harding 33, D Ford 33, S Beard 3-21, T Inness 3-22) lost to Marist 164-3 (C Thorpe 74no, M Inness 64no) by seven wickets in a 34-over match.