The final scores in Buenos Aires were written 6–4, 7–6 (7–1).
This has been an incredible week for me, said Fonseca after the match.
With the tournament victory, Fonseca, known for his rock-hard forehand and effective serve, climbs about 20 places in the ranking and will be the world's 68th best player from Monday.
Of course, I want to be number one, of course, I want to win Grand Slam titles. But my dream right now is just to play tennis, a dream I'm living in.
The Brazilian star's achievement has not gone unnoticed in the tennis world.
"Impressive, João", writes world number three and four-time Grand Slam champion Carlos Alcaraz on X.
Another who was impressed was former world number one Boris Becker.
"Look at this young man from Brazil. There are no limits to how far he can go."
Already on Tuesday, the next challenge awaits the 18-year-old. Then he will face Alexandre Muller in the home tournament in Rio, where Fonseca reached the quarterfinals last year.
