If ever a catch won a match then Suzie Bates' effort to remove Sophie Devine would be it.
The Sparks captain got under a towering shot from the classy Wellington batsman and held what was a tremendous catch under enormous pressure.
Devine was threatening to take the game away from the Sparks and, with it, their place in the final.
She perished for a hard-hit 37 and, while Liz Perry's knock of 42 took the game to the last over, it was Devine's wicket which swung the match.
Otago had posted a competitive 125 for seven with Katey Martin top-scoring with 42 from 44 deliveries.
Wellington needed 18 from the last two overs but could only add five runs in a tight penultimate over bowled by Leigh Kasperek.
Bates put the win beyond doubt when she bowled Perry, who had looked to lap the medium pacer.
The visiting side had to settle for 117 for five - an eight-run loss - but will get an opportunity for redemption.
Despite the loss, Wellington has qualified for the final which will be staged at the University Oval next Saturday.
The Black Caps have an one-day-international against Sri Lanka either side of the game.
Otago needed to win the match to make the final and Bates was thrilled with her team's effort.
''I'm just so pleased. We asked at the start of the day for everybody to contribute because that is what it takes to get into finals,'' Bates said.
''You have to put your hand up under pressure and I think we did that. We played really well under a lot of pressure.''
Devine had actually been dropped on 19 off Bates' bowling and she had wondered to herself whether that might be the game gone.
''We put her down which could have been a game-changer,'' Bates said.
''But we stuck with it with the ball and that catch at long-on was a bit of a game-changer. I was just lucky to be under it.
''I don't get to field out there that often so when I do I love it. It was coming a bit quicker than I thought.''
The Sparks knew instantly how big the moment was and celebrated accordingly.
Earlier, the Sparks made a terrible start when Morna Nielsen was bowled by a full toss in the opening over.
But Bates and Martin eased the nerves, adding 33 for the second wicket.
Bates never looked quite herself, fluffing the odd shot, and on 19 flicked a delivery straight to Rebecca Burns at short fine leg.
A metre either side and Bates would have scored four but equally, had she not played the shot, the delivery would have surely been called wide.
Potentially it was a huge breakthrough but Martin picked up the challenge in Bates' absence.
She played a couple of delightful shots off the back foot but on 16 was lucky to survive.
She mis-timed a drive but Perry dropped a sitter at mid-on.
Martin made Wellington pay, smashing consecutive fours in the 14th over and combining in a 44-run stand with Kasperek (30).
That partnership proved instrumental in getting her side through to a decent total.
Allex Evans bowled a superb spell for the visiting team, taking four for 13 and picking up the key wickets of Bates and Kasperek.
Wellington's reply slumped to 21 for three as Nielsen took two for 12 in a fine spell.
But Devine looked all class and attacked the short boundary.
Bates bowled from that end in an attempt to restrict her scoring but disappeared for an enormous six.
She should have picked up Devine's wicket when Polly Inglis shelled a chance at point.
When Devine departed, Perry responded with a counterattack.
She played some lovely shots but the scoreboard pressure kept mounting and eventually accounted for her wicket.
