Then, as Wainui regrouped to clear their lines, he misdirected a pass to his rightback to give away a corner, from which a Wainui defender conceded a penalty.
United midfielder Kieran Venema struck the spot kick firmly to the keeper's left, and Pierard redeemed himself by diving full-length to parry.
Wotherspoon's standout save came in the 42nd minute, when he sprang to his left to palm away a Thiago Carlos de Melo shot headed for the top corner.
Both keepers were sound in general play, and were called on often to leave their penalty areas to tidy up breaches of their teams' defences.
This was particularly important for Wainui, who deployed a relatively flat back four against the pace of Josh Adams and Cranswick.
It meant Pierard was up around the penalty spot watching for the through ball as United built their attacks. A couple of fruitless long-range shots aimed at catching the keeper off his line kept him from venturing too far from home.
It helped, too, that centrebacks Mal Furlan and Jamie Gallacher were on top of their game. Furlan's a speedster himself, Gallacher is no slouch, and they work together intelligently.
Furlan's finest moments came late in ordinary time when, in quick succession, he headed off the line and made a goal-saving tackle on Adams. Moments later, Pierard chipped in by tipping a rasping 30-metre drive by Venema over the bar.
Out wide, rightback Caleb Baldacchino and leftback Dan Torrie dealt with everything that came their way, which included forays to the wings by both Cranswick and Adams as they probed for openings.
Yates, usually strike partner for Cranswick, dropped into the midfield, where he did sterling work for United alongside Aaron Graham and Venema.
The middle of the park was a battleground where warhorses Shane Hooks and Michael Smith, for Wainui, and Yates, Venema and, venturing from the backline, Purcell, for United, ran and tackled themselves to a standstill.
Thoroughbreds like Max Logan and Martin Brusco, for Wainui, and Graham, for United, didn't shirk the hard yards, either. Their artistry was leavened by a liberal sprinkling of grit.
Hooks and Venema were awesome to watch. Both have had limited game time this year, mainly because of injuries. In the game's closing stages, they dragged their exhausted bodies back to defend every counter-attack.
United paid Wainui the compliment of defending in depth.
Purcell was almost an extra midfielder, forming a buffer for the other two central defenders, the rugged Matt McFatter and the fast and tenacious Lucian Nickerson.
Leftback Caleb Craig and rightback Ben Hansen, both quick, completed a typically tough United rearguard.
Wainui strikers Carlos de Melo and Jaiden Ibbetson struggled to get past the human wall confronting them.
The efforts of Carlos de Melo were rewarded with his 61st-minute goal that tied up the scores, and four minutes later he rapped the underside of the crossbar from 30 metres.
Substitutes Jake Theron, still hampered by injury, Nicolas Carrizo and Henri Campbell were brought on to refresh the attack but United remained resolute, despite the fatigue that became a factor.
United had opened the scoring in the 59th minute after a flowing passing move from Graham to Cranswick down the right wing and then to Adams for a first-time shot into the far corner of the goal.
The reply from Wainui, two minutes later, came when Logan contested a ball knocked from deep, into the penalty area, and headed it on for Carlos de Melo to nod in from five metres.
Four minutes into extra time, Brusco grabbed the winner. Carlos de Melo crossed from the left, and Brusco controlled the ball quickly and shot towards the far post. Craig, rushing back to guard the goal, could only help the ball in.
Wainui brought on John Hill to shore up the defence and United brought on Chris Cockburn for fresh legs.
A Graham volley was well saved by Pierard, a Logan shot was covered by Wotherspoon and, in almost the last act of the game, Brusco was sent off for a second yellow-card offence, this one for taking too long over a corner. Others to see the yellow card were Carlos de Melo and Campbell for Wainui, and Venema for United.
Referee Chris Niven kept the lid on a match that had pressure-cooker intensity but was played in good, competitive spirit.
Wainui co-coach Michael Smith said his team “dug deep”.
The addition of Venema and Adams from United's Pacific Premiership team had helped make the game the toughest of the season for Wainui, and they thrived on the challenge.
Brusco was “extraordinary”, Furlan and Gallacher were outstanding at the back, and Hooks — injured for most of the season — was “a rock”.
The team's success this year showed the benefit of good coaching, and he attributed much of that to Blake Mulrooney, who had worked in tandem with him.
“This is a great day for the club, and this club is growing,” Smith said. (Another club team, 1st Class Decorators Wainui Salty Dogs, won the Eastern League Division 2.)
United coach Chris Adams said his team had played “out of their skins”.
“They listened to our plan and stuck to it — we wanted to stop them coming down the wings and getting crosses in,” he said.
“I was pleased with the defence. Lucian (Nickerson) had a great game, and Aubrey (Yates) did well playing in midfield.”
Adams said he was pleased that United had managed to create as many goalscoring chances as they did.
