"Then found I had won ... to do this today, I can't believe it."
Sadowski-Synnott became the first reserve during the week when Austrian Anna Gasser pulled out. Hours before the competition, she found Slovakia's Klaudia Medlova has also withdrawn after suffering an injury in practice.
"You have my spot now," Medlova told the young Kiwi.
Sadowski-Synnott said: "The X Games for us snowboarders is the best of the best, an invite-only which has been around so long.
"Doing this today means so much to me, probably a career highlight.
"I honestly came here this week just wanting to get a feel for the X Games — I was only in the big air at that point and focussing on that.
She described the whole experience as "sick" and the buoyant crowd as "super sick".
"I'm just going to go and hang out with the New Zealand team and celebrate, get back and do some snowboarding," she said.
Sadowski-Synnott and fellow teen Nico Porteous won bronze medals at the Olympics last year, the first Kiwis to medal in 26 years.