That made territory the order of the day to ensure Whanganui was in position to attack if the visitors faltered, or they at least had a long way to go if they recovered the ball.
Skipper Dane Whale, try-scoring halfback Lindsay Horrocks and fullback Ethan Robinson went high and into the corners, hoping to make Thames Valley falter, while Thames Valley first-five Todd Doolan countered with cross kicks into Whanganui's defending 35m with the wind behind him in the first half.
The visitors initially struggled with dropped ball through their midfield early, while their lineout was countered well by Whanganui lock Josh Lane and his cohorts in the air.
But Whanganui weren't immune to juggling the ball either, while for a long time on either side of halftime they did not receive a penalty but the visitors got about five, thanks to pressure on the counter-ruck, which let Thames Valley close the gap to 12-11.
Turning with the wind, it appeared Whanganui's composure would see them home as, following some earlier misses, Robinson slotted close range kicks, and then a well-taken team-try by second-five Timoci "Jim" Seruwalu off a penalty lineout was followed by a smooth Whale drop goal in front for a 25-11 lead in the 76th minute.
But having won on their last two visits to Cooks Gardens, you write Thames Valley off at your peril, as they responded with outstanding ball control to get back from the 50m to having centre Sam McCahon scoring a converted try with a minute to play.
In a pulsating finish, Thames Valley went sideline to sideline in the hunt for seven more points to send the game into extra time and, finally, dangerous winger Harry Lafituanai charged clear down the far sideline with support trying to get to him.
The decisive play was Robinson's who, along with reserve halfback Caleb Gray, cut the winger down and then Robinson rolled up to rip the ball free, with Whanganui setting the ruck and booting the ball out for fulltime.
A scrappy encounter, the result was just reward for a Whanganui forward pack who delivered in the set-piece and were steady in the scrum all the while keeping the opposition lineout off balance.
Blazer game-playing lock Ben Whale and try-scoring second-five Jim Seruwalu made umpteenth hit-ups, while Lane overcame having double vision for nearly 20 minutes after a poke in the eye to still claim his lineout ball and clear bodies out off the breakdown.
Horrocks had a hand in everything and really pressured Thames Valley's replacement halfback Leroy Neels at the back of the scrum to stifle the visitors before they could get a roll-on.
Winger Alekesio Vakarorgo scored a stunning individual try in the first half where he beat four defenders, reserve centre Kameli Kuruyabaki helped Whanganui keep the tempo up during their scoring burst late in the second half, while No8 Semi Vodosese produced some threatening raids, although finishing them off in the face of determined cover defence proved elusive.
At the ruck, Thames Valley had the right men to do the hard stuff as well: try-scoring flanker Ben Stephens impressing until he came off injured, while fellow flanker Latuea Mau Fangufangu was a threat, and lock Cameron Dromgool got wide to provide Lafituanai the pass that nearly saw him get away.
However, the Swampfoxes forcing extra time would not have reflected the overall run of play – Whanganui looked the more threatening side, scoring three tries to two.
"We were one or two passes from blowing that out, probably, like three to four tries," said Dane Whale.
"It's confidence to know that [while] opportunities were probably missed, hey, that's finals footy – you're not going to hit everything.
"But I felt like we were throwing way more than they were, it actually felt like we were in control a lot of the time.
"The second 40 was massive. Kudos to [Thames Valley], they had a good game."
Coach Jason Hamlin was happy with that composure in big match pressure, although he naturally would have preferred the side to have landed a knockout blow before the grandstand finish.
"It's a semifinal, the tightness is always going to be there, it was always going to be that tight of a game.
"I thought we were trying to be too expansive. There were times I thought we could have just carried, been real direct, cleaned better and recycled.
"When we did that, and held some pill, they looked really good. When we were inaccurate and gave away ball real easy, that brought them into the game.
"They are a very good side, and they're well coached. That try before halftime, as much as I didn't like watching it, that was a beautiful try.
That spoke to the skill level in Heartland rugby, Hamlin said.
"We knew this was going to be that battle, and it was, as befitting a semifinal.
"Ten minutes to go, I was up pacing. There were times I was coming down the radio saying, 'we need another set of points', and Dane took that droppie that gave us that margin.
"Jim's try, just some good rugby. We probably won the little moments within it."
Whanganui got a couple of early penalties and although Robinson was away with his first kick, the hosts were soon back down there with an attacking lineout, and after several carries, Vodosese was all but pushed over, with Horrocks just picking it up and feeding the ball under bodies to place it on the tryline for 7-0 in 12 minutes.
But mistakes and penalties prevented Whanganui extending their lead, and Thames Valley chased a kick that turned into a holding infringement for Doolan to close the gap in the 26th minute.
Whanganui responded by winning the kickoff and although they lost the ball, they soon forced a turnover and then Vakarorogo wrong-footed his marker, dashed through another tackle and reached out through the last defender to slam the ball down in the corner for 12-3 with 10 minutes left to halftime.
But they could not ride that advantage into the break, as just two minutes later, Thames Valley isolated and counter-rucked to force a turnover, and after attacking down the grandstand sideline, the regular centre-turned-winger Lafituanai dashed on the angle from the midfield to the far side to put Stephens away clean into the corner.
Right on halftime, again Whanganui got isolated for a penalty, and Doolan closed the gap to a single point.
Doolan couldn't add a go-ahead penalty or a long range drop goal attempt after the break, as both teams struggled to maintain possession, but Horrocks produced a dart and offload to hooker Roman Tutauha to set up a chance, and although the final carry by prop Keightley Watson was held up, Whanganui had the penalty advantage and Robinson added the simple three-pointer.
While Robinson could not add a further three points from out wide, Whanganui stayed camped in Thames Valley's half, and another penalty saw Whale boot them right into the pocket.
From the lineout drive, with the backs approaching to join, Horrocks took it off the back with a jinking run towards a straight-running Seruwalu, who jogged untouched under the posts.
Up 22-11 with ten minutes left, Whanganui appeared to have sealed the game when they counter-rucked and their fresh reserves helped carry forward to the posts, where Whale slotted the drop goal dead in front.
But Thames Valley worked back into the attacking half and got a penalty to set for a lineout, with Fangufangu charging at the line, before the ball was transferred wide and McCahon stepped the cover and dived under the heap to score, with reserve Zac Stephens taking a quick conversion to set up the thrilling finish.
Whanganui 25 (L Horrocks, A Vakarorogo, T Seruwalu tries; E Robinson pen, 2 con, D Whale dg) bt Thames Valley 18 (B Stephens, S McCahon tries; T Doolan 2 pen, Z Stephens con). HT: 12-11.
