What followed reverberated far beyond Eugene, Oregon, and the U.S. Olympic trials.
By one-hundredth of a second, Olympic champion Felix, hampered by an ankle injury and a lack of training, lost to Prandini for the third and final 200 metres spot on the U.S. team for Rio.
The defeat ended any chance of her going for a rare 200-400 metres Olympic double since the American selection system is notoriously cut-throat, based solely on a top three finish in the trials.
"This whole year that had been what I was working for," the disappointed Felix said of the double.
Her goals now will be to win the 400 metres, where she is a favourite, and race in one or two relays.
"It's pretty amazing to me that I'm still going to Rio," the world 400 metres champion told Reuters Television.
"While it doesn't look exactly how I hoped it would, I'm still going after it with everything I have."
America's most decorated female sprinter, Felix had won her first Olympic medal, a silver in the 200 metres, as an 18-year-old at the 2004 Athens Games.
Blessed with the speed of a sprinter and the strength to endure the 400 metres, the now 30-year-old Californian has gone on to grab 18 more outdoor global medals - 13 of them gold.
The double never would have been easy, even with the International Association of Athletics Federations (IAAF) changing the Rio schedule to make it more plausible.
Dutch speedster Dafne Schippers and American Tori Bowie would be waiting in the 200 and Bahamian Shaunae Miller in the longer distance.
But Felix wanted to try despite the severe ankle sprain in late April that almost railroaded her Rio chances.
"Two months ago I couldn't even walk," said Felix. "Somehow we found a way."
Pool workouts and lots of physical therapy prepared her for the 400, but not just any 400.
Digging deep on the final straight, Felix powered home in the fastest 400 of 2016 to make her fourth Olympic team.
Coach Bob Kersee called it her greatest performance.
Felix, however, could not muster the tell-all speed in the 200, though it was not the ankle injury that affected her, but the lack of conditioning, she said.
"The speed just wasn't there," said Felix, who had not run a competitive 200 this year until the trials.
"I think my biggest issue with running with the injury was running out of time, really. Now that I have another month to get back to 100 percent I feel confident that I will be ready to go when it comes to Rio."
(Additional reporting by Reuters Television in Los Angeles; Editing by Greg Stutchbury)
