The 17-year-old Kouamé held firm in a contest lasting just under five hours on a sun-drenched Court Suzanne-Lenglen. He clawed back from 5-3 down in the fifth set to force a decisive tiebreaker and rallied again from 8-7 down in the tiebreaker.
He won with a drop shot that his Paraguayan opponent got his racket to but could only pat into the net.
Kouamé dropped on to his back and, when he got back up, put both hands on his head in disbelief. Then he thumped his chest and pointed to all sides of the crowd.
"It's something I've always dreamed to do, get the crowd going and making a bit of a show on court," he said.
"I'm really trying to understand when is the right moment to feel the most energy the crowd is giving me."
The No. 318-ranked Kouamé was overcome with emotion as the victory sank in. He slumped back in his chair with a towel on his head as joyful fans chanted "Merci Moïse, Merci Moïse," at Roland Garros.
Before his on-court speech, Kouamé bent over with cramps and emptied a water bottle over his head.
"This win is for you," he told the crowd. "Thank you because without you, honestly, I would never have won this match. ... You don't even realise how much you carried me with the way you kept shouting for me. When I found myself 5-3 down in the fifth, I never stopped believing."
Kouamé next faces Chilean Alejandro Tabilo and is thinking big.
"Winning Roland Garros is, of course, a dream," he said. "But winning all four (majors) is a dream actually."
Kouamé beat Marin Cilic in straight sets in the first round on Tuesday, becoming the first man born in 2008 or later to win a Grand Slam match.
Cilic won the 2014 US Open and reached the French Open semi-finals in 2022. He said he was impressed with Kouamé's ability to stay focused at key points in their match.
Kouamé has been working on mental strength with his team, which includes former player Richard Gasquet, a three-time Grand Slam semi-finalist,who is in his coaching staff.
"This is a major thing. It's really important when the level is higher to play the best you can on important points," Kouamé said. "The small details that make me win or lose the match. I'm really happy (Cilic) says that I'm pretty strong, because I've put a lot of hours on training on that aspect."
His coach Liam Smith expanded further how he has helped Kouamé make the transition to the senior tour.
"When you move from being a young player from the juniors or the lower-tier events to the bigger stages you see a lot of rookie mistakes or junior mistakes or lapses of concentration," Smith said.
"That's something we have been working on a lot for the last month or so, just to improve the ability to stay more disciplined."
In March, Kouamé was the youngest match winner in Miami Masters history in the first round, earning a congratulatory message from 24-time major winner Novak Djokovic.
9°C