[4][5] His father is CEO and co-founder of IP Capital Partners, the first independent hedge fund in Brazil.

[6] Fonseca began playing tennis at the age of 4 at the Rio de Janeiro Country Club, which was next to his home.

In January 2024, the 17-year-old Rio native reached the semifinals of the Buenos Aires Challenger, his first in his career in this type of tournament.

655, he made a second appearance in the main draw at the 2024 Rio Open after receiving a wildcard in singles and also in doubles qualifying with Marcelo Zormann.

[20] Days after the end of his run in Rio, Fonseca was rewarded with a second consecutive ATP Tour tournament wildcard at the 2024 Chile Open.

[21] Later that week, he officially announced his decision to fully turn pro, forgoing his eligibility to play college tennis and ending his commitment to the University of Virginia.

[22] In March, at the 2024 Paraguay Open, Fonseca defeated Argentine Román Burruchaga to reach his first ATP Challenger final and made his top 300 debut at world No.

276, he received a wildcard for the 2024 Țiriac Open in Bucharest, Romania and reached the quarterfinals of an ATP Tour event for the second time, defeating sixth seed Lorenzo Sonego, his third ATP tour win,[25] and fellow qualifier Radu Albot.

[27] Fonseca received a wildcard for the main draw of the 2024 Madrid Open, making his debut at a Masters 1000 and defeated American Alex Michelsen,[28] recording his first win at this level.

[37] Fonseca lifted his second Challenger title at his very first tournament of the season, the 2025 Canberra Tennis International, defeating American Ethan Quinn in the final,[38] recording his 10th consecutive win and reaching his new career high ranking at No.

[39][40] He defeated ninth seed Andrey Rublev in straight sets in his first major main draw match for his first top 10 victory,[41][42] becoming the first teenager since Mario Ančić (against Roger Federer in 2002 Wimbledon Championships) to beat a top 10 player in the first round of a Grand Slam.