Joao Fonseca was born on August 21, 2006, in Rio de Janeiro, Brazil. His parents are Roberta and Christiano, who both played several sports and influenced Fonesca. During childhood, Fonseca was inspired to play tennis by attending several Rio de Janeiro ATP Tour competitions. He also started playing at the Rio de Janeiro Country Club.
Fonseca had a stellar junior career and won five junior-level titles between 2021 and 2023. He also reached the boys' US Open final in 2023 and beat Learner Tien. At the end of the 2023 season, Fonseca was ranked number one in the world at the junior level.
Earlier in 2023, the Brazilian also made his ATP Tour debut. This came at his home tournament in Rio and he entered on a wildcard at age 16, but he lost in the first round. However, one year later in 2024, Fonseca won his first-ever ATP level matches in Rio and progressed to the quarter-final. He then lost against Mariano Navone in the final eight.
Fonseca built on his confident showing in Brazil and reached further landmarks in 2024. He progressed to the Paraguay Open Challenger final before losing to compatriot Gustavo Heide in the final. Another ATP-level quarter-final run at the Țiriac Open in Bucharest followed.
In Romania, Fonseca beat Lorenzo Sonego and Radu Albot before losing against Alejandro Tabilo. In April, Fonseca received a wildcard for his first-ever Masters level competition at the Madrid Open. The Brazilian player kept producing strong form and won in the first round. He progressed past Alex Michelsen and climbed up to 226th in the ATP Rankings.
