Miller, the 22-year-old from the Bahamas, stayed even with Felix for 398 meters, then sprawled, dove and crashed across the line to edge Felix by .07 seconds.
She'll get the gold medal in the 400 meters. Maybe they should give her a cape, too.
This was supposed to be a stroll and something of a coronation for Felix, who was the defending world champion and had the best career time of the eight women in Monday night's final. She was trying to become the first woman to win five track golds at the Olympics.
Halfway through the race, it was clear that was no sure thing.
Starting from Lane 7, Miller expanded the lag, instead of getting gobbled up. Felix slowly chipped away. They came down the last 100 meters and Felix drew even, maybe even got a step ahead.
Stride for stride they ran, until the last few steps. Felix, classically trained by Bobby Kersee, made a textbook lean into the finish line. Miller tried something else. The dive is something no coach would ever teach. Then again, amazing things happen with a gold medal on the line.
As Miller lay on her back, writhing in agony, Felix sat on the ground stone-faced. Ten seconds passed. Then 20.
The rules say the win is determined by which athlete has any part of her torso cross the line first. The photo finish showed the negative image of Miller's sprawled out body, with her shoulder just barely over the line before Felix reached.
Finally, the result popped up. Miller won in 49.44 seconds.
What a finish. What a race.
And, yes, what a disappointment for Felix, whose year just didn't turn out the way she planned it.
She was one of those rare athletes who had the cachet to get the Olympics to change the schedule. After winning the world championship at 400 meters last year, she put the 200-400 double in her sights for the Olympics. The schedule as it was originally written made it impossible -- with the 200 heats scheduled for the same evening as the 400 final.
Felix asked, and she received: The 200 heats were moved to the morning to give America's best female sprinter a chance for the two-fer.
But she never got to the starting line in the 200. She landed awkwardly on a medicine ball while doing core work in the gym during the spring. "I'd never seen my ankle that big before, and it happened immediately," she said.
Suddenly, the quest for two golds was simply a struggle to make the Olympics. She did in the 400. But the 200, which requires more "speed work" running hard around the curve, never got where it needed to go. Felix came in fourth at trials, one spot out of the mix, and had to swallow that disappointment and get ready to make the 400 hers.
Miller had a different plan.
The flagbearer for her country in the opening ceremonies, Miller came into the games 5 for 5 in her races this season, including Diamond League meets in Shanghai, Eugene and London.
Now, she's 6 for 6.