Joao Fonseca

Fonseca reached the final of the boys' doubles at 2023 Australian Open Jr. partnering with Belgian Alexander Blockx. Later on that year, he won the boys' singles title at the 2023 US Open with a victory over American Learner Tien in the final. He became the third Brazilian player major jr. champion, after Tiago Fernandes and Thiago Seyboth Wild.

Fonseca was named the 2023 world champion of the Junior circuit. At 17 years old, he was the first Brazilian to finish the season as No. 1 in the ITF junior rankings.

He 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.

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. 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, 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. 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, 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, 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. 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.

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.

He 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.

João 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 2025 BNP Paribas Open tournament and lost in second round to the thirteenth seed and eventual champion Jack Draper. This early defeat led him to play the following week in the 2025 Arizona Tennis Classic, where he won his second singles Challenger title (biggest of the series at 175) for the season and fourth overall. He became the first player to win an ATP 250, Challenger 175 and Challenger 125 trophy in the same season.