The 19-year-old powered his way to a 7-5, 7-6 (10/8), 5-7, 6-2 win in the night match at Roland Garros to reach his maiden Grand Slam quarter-final, where he will face Czech youngster Jakub Mensik.
Fonseca showed a different side to his game after coming back from two sets down against both Dino Prizmic in round two and Djokovic on Friday.
He held off a battling Ruud, a two-time French Open finalist, in a gruelling contest that lasted three hours and 55 minutes and finished at 12:27 am local time on Monday morning.
“It was tough, Casper plays good here, he’s a very experienced guy and he knows how to play here on this court,” said Fonseca, who hit 51 winners in another exciting display.
“It was tough in the beginning but I played well in the important moments in the first and second sets.”
After Rafael Jodar’s victory over Pablo Carreno Busta on Sunday, Fonseca’s win means it is only the fifth time in the last 40 years two teenagers have reached the quarter-finals of a men’s Grand Slam competition.
He is the fourth Brazilian man to reach the Roland Garros last eight in the Open era, and the first since three-time champion Gustavo Kuerten, who was watching on from the players’ box on Court Philippe Chatrier.
“I just try to be me on court, try to be happy, try to hit winners, try to hit good shots, try to be entertaining,” added Fonseca, the 28th seed. (AFP)