The final had been delayed by more than an hour after a heavy blizzard set in at Livigno Snow Park, while Mathilde Gremaud, a heavy favourite for a medal, was one of two Swiss skiers to withdraw last-minute through injury.
That looked to have opened up the field for Muir, who had qualified in fourth for the final.
But in an astonishing first round, four skiers posted scores of 90.00 points or more, with Muir languishing in seventh and knowing she needed to go big.
She did just that, posting 93.00 with a 1620 trick, featuring four and a half rotations, the highest score of the second run and one that catapulted her up the standings into silver medal position.
At that point Gu, already a silver medallist in the slopestyle at these Games, was way off the pace having struggled with her second attempt, but she made amends on her final jump to bump Muir down into third.
The Chinese skier celebrated like her medal was confirmed at that point, despite plenty of skiers waiting in the wings to nudge her off the podium.
As it proved, they couldn’t do that, though Tabanelli’s final jump of 94.25 points – the biggest score of the night – came just 0.75 points shy of silver medal position.
That piled the pressure on Muir’s third and final jump.
She took her time at the top of the big air structure, talking through her options with her coach and decided to go for another 1620 trick with a different grab, but ultimately could not land her effort, leaving her lost in her thoughts of what might have been as she sat on the snow.
“When the scores came in for the other girls, I knew I had to give it something really, really good to try and get on that podium, so I’m stoked that I did try that,” she said.