Thiago has three sisters (Letícia, Jéssica, and Flávia) and an older brother, Fáber Monteiro, who works as a real estate broker.

[10] Thiago would leave his hometown, Fortaleza, in late 2008 to enter the Larri Passos Academy in Balneário Camboriú to improve his game.

He won his third and final title at the 40th edition of the Banana Bowl, this time in the u16 category, and also made his debut on the ATP Singles Rankings, with his participation in a Future in his birthplace, in Fortaleza.

In 2011, Thiago started the year winning the Grade A Copa Gerdau in Porto Alegre, one of the most important tournaments on the ITF Junior Circuit.

[17] Thiago didn't achieve much success in singles Grand Slams as a junior: his best results were reaching the third round at the Wimbledon Championships in 2011 and 2012 French Open.

[19] This win led Thiago to receive a wildcard entry on the main draw of Aberto de São Paulo, but he was eliminated on the first round by compatriot André Ghem.

[21] The Brazilian was runner-up on two other Futures played in Brazil: at São José do Rio Preto in August[22] and at Porto Alegre in October.

He also won his first Futures title in doubles, partnering Maximiliano Estévez, from Argentina, defeating in the finals the partnership of Kirill Dmitriev and Yaraslav Shyla in the super tiebreak.

His best results were in the quarterfinals in Germany and Netherlands, a performance he also achieved in IS Open and Aberto Rio Preto, but he didn't need to play the qualifiers in the latter.

He also reached the semifinals of IS Open and Aberto Rio Preto, partnering Thiago Alves in the first and Rogério Dutra Silva in the latter,[27] achieving his career high in the doubles ranking: 449th.

Partnering with the Italian Marco Bortolotti, the left-hander won his 2nd Futures title by defeating the Russian partnership of Victor Badula and Ivan Kalinin in straight sets.

[36] Almost a month later, playing in the qualifiers of Poprad-Tatry ATP Challenger Tour, Thiago badly injured his left knee while holding a match point against Czech Robin Stanek[37] and was forced to retire.

463, Thiago started strong, qualifying to Torneo de Mendoza main draw and reaching the quarterfinals, where Monteiro lost to eventual champion Gerald Melzer in straight sets.

[39] This performance led him to receive a wildcard into Rio Open main draw where he would face Jo-Wilfried Tsonga, the third seed and world No.

[47][5] Returning to Europe, Thiago entered the Open du Pays d'Aix draw unseeded, and with a strong performance, he conquered the title by beating Carlos Berlocq in the final.

Playing in another clay tournament in France, Monteiro reached a second Challenger final, this time at the Open Sopra Steria de Lyon.

[60] Thiago also did his debut in doubles in Atlanta, playing with Yoshihito Nishioka, but they lost at the super tiebreak against American partnership of Zack Kennedy and Christopher Eubanks.

[61] A week later, Thiago entered directly into the main draw of an ATP event for the first time, the Los Cabos Open; but lost in the first round to Dušan Lajović in straight sets.

[62] Thiago finally broke the Top 100 milestone after winning in the first round of the Western & Southern Open qualifiers against veteran Ivan Dodig in straight sets.

Thiago ended his year with two other quarterfinals (Campinas and Buenos Aires) and a semifinal (Santiago) appearance at Challenger events.

Playing on Argentinian soil, Monteiro passed the first round, defeating ex-top 10 Tommy Robredo en route to the quarterfinals of the Argentina Open, before losing to local player Carlos Berlocq in three sets.

[73][74] Monteiro made a consecutive quarterfinal appearance, this time at the ATP 500 Rio Open, beating Gastão Elias and compatriot Thomaz Bellucci before losing to Casper Ruud.

[79] At the maroon clay courts, Thiago defeated seventh seed Donald Young in straight sets but lost in three to Ernesto Escobedo.

Thiago played the qualifier where he won the first game and lost in the second but the Luck allowed him to compete in the main draw as a lucky loser.

He won 3 Challengers titles in Punta del Este, Braunschweig, and Lima, in addition to making the semifinals in Santo Domingo and Buenos Aires.

Monteiro reached the first Grand Slam third round in his career at the 2020 French Open defeating 31st seed Nikoloz Basilashvili,[85] and Marcos Giron before he lost to Márton Fucsovics.

He reached the quarterfinals of the ATP 250 in Adelaide, after winning 2 qualifying games and defeating Gael Monfils in the 2nd round of the main draw.

[88] In July 2022, Monteiro won his seventh Challenger in Salzburg, beating Norbert Gombos in the final, reaching a career-high of world No.

In September, Monteiro began his recovery, where he reached the semifinals of the Challenger 125 in Genoa, Italy, and the following week, participating in the Davis Cup, he obtained one of the biggest victories of his career, defeating world No.4 Holger Rune in Denmark, on hardcourts, helping Brazil win the match 3-0.

[113] Being the team singles No.1 for a second time, this turn against the Dominican Republic, Monteiro failed to confirm his ranking superiority, losing his second match against No.184 José Hernández-Fernández after winning against Roberto Cid Subervi.