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The 20-year-old freeskier defended his superpipe total with the last run of the entire event.
"I can’t believe it," Porteous said after he sealed victory.
"Another one? Like, what? I am genuinely speechless."
Porteous was third heading into his fourth and final run, and delivered what commentators described as the most technical halfpipe run ever landed in competition.
He dropped in switch starting with a switch right 900, into a newly learnt switch left double 1440, then into his trademark back-to-back double 1620s before finishing off with a huge left alley oop double 900.
"I was the last to drop so it happened so fast.
"To piece all that together in arguably the most high-pressure moment of my skiing career, I can’t believe it."
His brother, Miguel Porteous (22), finished seventh at his fourth X Games appearance.
Another Wanaka freeskier, Ben Harrington (20), dropped into his debut X Games and finished in a creditable eighth place.
It was the first time the Porteous brothers had competed alongside each other at the X Games.
American Aaron Blunck claimed the silver medal in his 12th X Games appearance and fellow American David Wise, a two-time Olympic gold medallist, won bronze.
There was one final honour for Sadowski-Synnott.
After the superpipe, X Games organisers announced she had been awarded the "best in snow" Golden Grill award for winning two gold medals and becoming the first woman to land back-to-back double corks in a slopestyle run.
"Honestly, words can’t describe it — I am so stoked," Sadowski-Synott said.
"Seeing Nico lace the best run of his life tonight tops it off."
Both Wanaka stars will now head to Beijing with the New Zealand team for the Winter Olympics.
They each claimed bronze in Pyeongchang four years ago and are genuine chances to win New Zealand’s first gold medal in the history of the Games.
-- STAFF REPORTER