2023: First professional matches, ATP Tour debut Fonseca made his ATP debut at the
2023 Rio Open after receiving a wildcard for the singles main draw and also appeared in the doubles main draw, entering as lucky losers with
Mateus Alves.
2024: First ATP wins, NextGen Finals title In January 2024, the 17-year-old Fonseca reached the semifinals of the
Buenos Aires Challenger, his best performance in this type of tournament. Until then, Fonseca had reached the quarterfinals in two challengers, the first in 2022 in São Leopoldo and the most recent in 2023, in Florianópolis. Ranked No. 655 that moment, Fonseca 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. He recorded his first ATP and first ATP 500 win over seventh seed
Arthur Fils in straight sets, giving away only four games. Excluding the Davis Cup, he became the first South American to claim an opening set 6–0 against a top 50 ranked opponent before turning 18, since the ATP rankings were first published in 1973. He also became the first player born in 2006 to win an ATP Tour match. Next, he defeated
Cristian Garin in straight sets to reach his first ATP quarterfinal. As a result, he moved 300 positions up, making him the youngest player in the top 350 in the rankings. He became the second youngest ATP 500 quarterfinalist since the series began in 2009 and the first since
Alexander Zverev made the last eight in Hamburg in 2014. 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. 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. 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. 288 on 1 April 2024 moving more than 50 positions up in the rankings. He lost to compatriot
Gustavo Heide in the championship match. Ranked No. 276, Fonseca 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, and fellow qualifier
Radu Albot. He lost to fourth seed
Alejandro Tabilo. As a result, he reached the top 250 in the rankings. 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, recording his first win at this level. Ranked No. 217, at the beginning of the grass season, he also received a wildcard for the main draw of the
2024 Halle Open. He reached the top 175 at world No. 166 on 5 August 2024, climbing close to 50 positions up, following lifting his maiden Challenger title at the
2024 Lexington Challenger, without losing a single set in the tournament. At 17 years old, he was the youngest Challenger champion of the 2024 season. Fonseca qualified for the
2024 Next Generation ATP Finals on 29 November 2024. Although the lowest ranked player, he won the title.
2025: Major debut & third round, ATP 500 title, Brazilian No. 1, top 25 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, recording his 10th consecutive win and reaching his new career high ranking at No. 113 on 6 January 2025. . Fonseca
qualified for the
Australian Open to make his major debut. He defeated ninth seed
Andrey Rublev in straight sets in his first major main draw match for his first top 10 victory, 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. Fonseca made his Top 100 debut on 27 January 2025, at No. 99. At 18 years 5 months and 6 days, he became the youngest Brazilian to ever do so, surpassing
Cássio Motta. Fonseca became the youngest Brazilian in the
Open Era to reach an ATP semifinal, at the
2025 Argentina Open and the first man born in 2006 or later to reach a final. At 18 years 5 months and 26 days, Fonseca became also the tenth-youngest tour-level finalist since 2000, and the youngest South American finalist since former World No. 20 Argentine
José Acasuso in 2001. With reaching the final he entered the top 75 in the rankings, becoming the Brazilian No. 1 singles player on 17 February 2025. He lifted his maiden tour-level title with a straight-sets win over
Francisco Cerúndolo becoming the youngest Brazilian to win an ATP title in the Open Era. He was also the youngest South American champion and 10th-youngest titlist in ATP Tour era (since 1990). After that campaign, Fonseca came back to his hometown event at the
2025 Rio Open, where he played in the singles draw and lost to
Alexandre Müller in the first round. Fonseca received a wildcard to the
Indian Wells Open, a prestigious Masters 1000-level event. The Brazilian won against
Jacob Fearnley in three sets and lost in the second round to the 13th seed and eventual champion
Jack Draper. This early defeat led Fonseca to play the following week in the
Arizona Tennis Classic in
Phoenix, where he earned his second Challenger singles title – biggest of the series at 175 – for the season, with victories over fourth seed
Jan-Lennard Struff, former Top-10
Kei Nishikori and
Alexander Bublik in the final. It was also his fourth Challenger title overall. With this trophy, Fonseca became the first player to win ATP 250, Challenger 175 and Challenger 125 titles in the same season. At the
2025 Wimbledon Championships, on his debut at the tournament, Fonseca became the first Brazilian man to reach the third round since
Thomaz Bellucci in
2010, and the youngest man to reach the third round since
Bernard Tomic in
2011. As a result he reached the top 50 on 14 July 2025. After reaching the third round of the Cincinnati Masters 1000, he reached No. 44 in the world, joining
Flávio Saretta as one of the top 10 Brazilian men's tennis players in history. He also reached the second round of the US Open, where he was eliminated by No. 22
Tomáš Macháč, and rose to No. 42 in the rankings. At the
2025 Swiss Indoors in Basel, he defeated defending champion
Giovanni Mpetshi Perricard in the first round. After a walkover from
Jakub Menšík, Fonseca then defeated world No. 23 and ninth seed
Denis Shapovalov in the quarterfinals and world No. 42
Jaume Munar in the semifinals. Fonseca was crowned champion after his victory over No. 18 and eighth seed
Alejandro Davidovich Fokina in the final. As a result of that title, Fonseca became the first Brazilian singles player to win an ATP 500 tournament since the tournament's designation was created in 2009 and also the second youngest player to lift a title at this level after
Carlos Alcaraz. That new title boosted his singles ranking to a career-high of world No. 28 on 27 October 2025. It was the biggest title of Fonseca's career to date, and the most notable achievement in Brazilian tennis in men's singles since
Kuerten's title at the
2001 Cincinnati Masters.
2026: First ATP doubles final and title In January 2026, Fonseca was scheduled to start the season by playing at the
Brisbane International and face
Reilly Opelka on the first round, but due to an injury on his back, he withdrew of that tournament. He would also play at the
Adelaide International the following week, but still feeling that injury, he also decided not to play. Later that month, he participated at the
Australian Open as the 28th seed, but lost on the first round to
Eliot Spizzirri in four sets. Fonseca's next tournament of the season was at the
Argentina Open in
Buenos Aires, where he was the defending singles champion, but could not defend his title due to a loss on the first round to
Alejandro Tabilo in three sets. His next tournament was at the
Rio Open, his hometown event, where Fonseca got his first win of the season by beating fellow countryman and qualifier
Thiago Monteiro. He lost on the second round to Peruvian
Ignacio Buse in three sets. Fonseca also received a wildcard to participate in the doubles event, partnering with compatriot
Marcelo Melo. The Brazilian pair reached the final, facing German
Constantin Frantzen and Dutch
Robin Haase. That was the first time on Fonseca's career that he reached a doubles final on any ATP tournament. Fonseca and Melo ended up winning that final, securing his first ever doubles title on the ATP Tour. As a result, Fonseca also achieved his best doubles ranking at that moment, No. 160. Fonseca arrived at the
Indian Wells Open unseeded. Despite this disadvantage, he collected victories against 16th seed
Karen Khachanov and 23rd seed
Tommy Paul to reach the Round of 16, where he lost to world No. 2
Jannik Sinner in two tie-breaks. ==Personal life==