The dynamic 19-year-old Fonseca, who's long been touted as a potential superstar in the sport's new age, demonstrated rare heart and talent to come back from two sets down to outlast the ultimate champion, 20 years his senior, 4-6 4-6 6-3 7-5 7-5 in a wonderful contest lasting seven minutes under five hours.
Magnificent in trying to combat Father Time in the draining heat and magnanimous in defeat, Djokovic's loss means his wait to surpass Margaret Court's 24 titles must wait, though this had felt like the best chance in his twilight years with Carlos Alcaraz injured and Jannik Sinner sensationally kayoed.
Not that he would admit to that. When asked whether he'd allowed himself to dream after Sinner's exit on Thursday, he snapped: "I don't care. I don't care. I'll stop you right there. No. Just lost third-round. Let's just talk about something else. Thank you."
Asked if he'd be back next year, he shrugged: "I don't know" but he certainly demonstrated the sort of excellence, especially in the first two sets when he gave Fonseca a bit of a clay-court lesson, that everyone expects he'll be back at 40.
Yet even if he wouldn't entertain the idea of the historic nature of his loss after being beaten in a grand slam by a teenager for the first time, he was happy to salute the kid who had withstood the best he could throw at him, saying: "Hopefully, he can be the next great thing.
"Talent, he's got firepower, he's got the whole Brazilian nation cheering him, so there's a lot of excitement around him, rightfully so. Hopefully he can win slams, definitely wish to see that."
For this was the day another superstar for the sport really announced himself, a Brazilian roared to the rafters by the French crowd because he was playing like a musketeer, flashing 68 winners. Djokovic hit 70 himself in a truly great match.
"It was just amazing from his side," admitted the 39-year-old. "Obviously not great for me to be facing a player playing in such level, but I don't think I've done too much wrong. It's just that he was just better.
At two sets down, it hardly looked that way as the third seed enjoyed complete control, but Fonseca stepped it up to wrest the third and broke early in the fourth, drawing huge cheers from his ever-growing Brazilian army of fans, with a series of his blistering forehands drawing gasps from the Philippe Chatrier faithful.
In the decider, he looked on his way to bowing out when Djokovic went 3-1 up but again hit back and broke decisively for 6-5 before closing out a landmark victory with three-straight aces. "I just believed that I could do the aces, it was crazy, I have never done that before. I felt like John Isner!"
