But the brilliant boy from Brazil is not the only teen shaking up the old order, with Rafael Jodar, born just a month later than Fonseca, ensuring there'll be two precociously impressive 19-year-olds in the last-eight after his magnificent comeback triumph over experienced Spanish compatriot Pablo Carreno Busta.
In another thrilling late-night contest on Court Philippe Chatrier on Sunday, Fonseca was, if anything, even more spectacular than in his changing-of-the-guard ousting of Djokovic as he had to repel the doughty Norwegian's rearguard action before prevailing 7-5 7-6 (10-8) 5-7 6-2 after midnight.
Ruud, the 15th seed, hung on grimly, seemingly exhausted by constantly having to try to extinguish the forehand rockets exploding off Fonseca's racquet.
The three-time slam finalist dug deep to cling on in set three before Fonseca threw in perhaps his worst service game of the match to gift him a way back, but the young world No.30 regrouped and, if anything, was more focused in the fourth as the Norwegian looked clueless at how to stop the barrage of winners - 51 in total.
Cheered on by Brazil's former three-time Paris champion Gustavo Kuerten, Fonseca admitted to feeling inspired by having his idol 'Guga' rooting for him.
Fonseca's "feeling tired" after his exhausting week's work, including two five-set comeback wins over Djokovic and Dino Prizmic, with the Ruud match lasting five minutes under four hours too.
But he looks a genuine contender and is a big chance in the quarters against Jakub Mensik, the Czech who recovered form a second-round on-court collapse to beat Alex de Minaur next time out and who again appeared to ailing physically while somehow outlasting Andrey Rublev 6-3 7-6 (8-6) 4-6 2-6 6-3.
Alexander Zverev, now a warm favourite, knows he'll have to next repel the "incredibly special" talent of Jodar after the Spaniard, who'd trounced de Minaur in Madrid, came from two sets down to concede just five games over the final three stanzas for a 4-6 4-6 6-1 6-2 6-2 triumph over 34-year-old Carreno Busta.
Now the lanky kid from Madrid, who was ranked 707 in the world a year ago and 165 at the start of this year, faces a quarter-final against Zverev, who may well never have a better chance to shed that irritating tag of 'best player never to win a men's slam' in the absence of Jannik Sinner, Carlos Alcaraz and Djokovic.
But the world No.3 isn't entertaining any discussion on whether he's now an outstanding favourite.
"I will give the same answer I gave two days ago," said the second seed after he'd disposed of Dutch lucky loser Jesper de Jong 7-6 (7-3) 6-4 6-1. "I will focus on the matches that are ahead of me. This is the only thing that I can control."
He sounded a bit less chilled than Jodar, who's evidently just having a ball.
"I'm trying to enjoy the moment," said the teen, who's now up to No.22 in the live rankings.
"I was talking to my friends and to my family before the match. It's a gift, to play a fourth round in a French Open! So my goal was to enjoy it."
Zverev is clear of the danger he poses, of course.
"He's somebody that can accelerate the ball from both sides," shrugged the German, "which is incredibly special."
