The Black Ferns have recovered from a debilitating early red card to record a 39-17 comeback win and clinch the Pacific Four title in Canada.
With 11 changes to the team that crushed Canada last week, the NZ women's inexperience showed, as mistakes and indiscipline saw the underdog Americans bolt to an early lead, their cause done a huge favour by the early exit of NZ halfback Iritana Hohaia.
Hohaia's first test start lasted less than three minutes, after she was given her marching orders for direct head-to-head contact in a tackle.
The highly physical Americans wasted no time capitalising, and when the Fern went into the sheds facing a 17-5 deficit and a further player down, due to a yellow card to No.8 Liana Mikaele-Tu'u, it appeared a monumental upset could be brewing.
Boosted by by the injection of captain Ruahei Demant and the veteran cavalry off the bench, the Ferns class shone through in the second stanza, as they ran in 34 unanswered points to close out the tournament unbeaten - despite playing 68 minutes with just 14 players and 10 with 13.
The Kiwis finished with a six-try-to-two edge against the US, who had beaten them just once in 15 previous contests. In their last encounter at the 2022 Pacific Four, New Zealand recorded a 50-6 drubbing at Whangārei.
The result also extends the Ferns' undefeated streak to 15 games.
Hohaia's departure forced coach Allan Bunting to look to his bench early, slotting Demant into into first-five and shifting No.10 Rosie Kelly into halfback.
The US immediately took advantage of their one-woman advantage, with Liz Tafuna breaking clear from the ensuing lineout to score the game's opening try.
Moments later, the player ledger was momentarily squared, after Georgie Lilly Perris-Redding was shown a yellow card for dangerous contact.
From the resulting penalty, hooker Grace Gago burst onto a short ball from the rolling maul and powered over to celebrate her test debut with a try.
Mistakes and indiscipline continued to cost the New Zealanders momentum, affording the US plenty of prime field position.
That soon took its toll again, when No.8 Rachel Johnson bulldozed through some weak defence to increase the Americans' unlikely lead to 14-5 in the 25th minute.
The situation worsened for the Black Ferns moments before halftime, when Mikaele-Tu'u was shown a yellow card, after repeated infringements, while defending their tryline.
That reduced New Zealand to just 13 players and the US slotted a penalty to extend their advantage to 17-5 at the interval and leave the world champs a mountain climb to avoid an upset.
The Black Ferns had to be the first on the board in the second half - and they were. Wing Kelsey Teneti pierced a hole in the US defence, then found substitute halfback Arihiana Marino-Tauhinu for their second try.
A few minutes later, Renee Holmes slotted a penalty to reduce the deficit to just two points, before Phillipa Love barged over from close range to give New Zealand their first lead of the game.
With the floodgates opened, Mikaele-Tu'u brushed aside tacklers from a lineout for the Ferns' third try, followed closely by another from Demant to put the result beyond doubt.
In the dying stages, outstanding midfielder Amy du Plessis added a cherry on top with the Ferns sixth and final try in the dying stages.
With both Pacific Four and O'Reilly Cup locked up, the Ferns will return home to break for the Farah Palmer Cup, before they face Australia at Hamilton on September 30.
Black Ferns 39 (Gago, Marino-Tauhinu, Love, Mikaele-Tu’u, Demant, Du Plessis tries; Holmes penalties, 2 conversions) USA 17 (Tafuna, Johnson tries; Hawkins penalty, 2 conversions)
