At the 2010 Pan Pacific Swimming Championships in Irvine, California, United States, Fratus made his first major involvement in an international tournament, finishing 4th in 50 metre freestyle, with a time of 21.93 seconds.

[8] In the 2011 World Aquatics Championships held in Shanghai, China, Fratus made the fastest time in the semifinals of the 50 metre freestyle: 21.76 seconds.

[13] In April 2012, disputing the Maria Lenk Trophy in Rio de Janeiro, Fratus earned the mark of 21.70 seconds in 50 metre freestyle.

[19] At the 2014 Pan Pacific Swimming Championships in Gold Coast, Queensland, Australia, Fratus won a bronze medal in the Brazilian 4×100 metre freestyle relay, along with João de Lucca, Marcelo Chierighini and Nicolas Oliveira.

He won the gold medal in the 4×100-metre freestyle relay, together with Matheus Santana, João de Lucca, and Marcelo Chierighini.

César Cielo did not swim the final – despite being participating in the championship, he was feeling shoulder pain on this day;[33][34][35] and 6th in the 4 × 100 metre mixed freestyle relay, along with Matheus Santana, Larissa Oliveira and Daynara de Paula, breaking the South American record with a time of 3:25.58.

[37] At the 2016 Summer Olympics in Rio de Janeiro, Brazil, Fratus arrived as one of the favorites to win a medal in the Men's 50 metre freestyle.

The Brazilian Aquatic Sports Confederation (CBDA) even sent a physiotherapist to the United States, where Fratus lives and trains to accompany the athlete.

[38] At the 2017 World Aquatics Championships in Budapest, in the Men's 4 × 100 metre freestyle relay, the Brazilian team composed of Fratus, César Cielo, Marcelo Chierighini and Gabriel Santos achieved a historic result by winning the silver medal, the best Brazilian result of all time at World Championships in this race.

Brazil beat the South American record of 2009, still in the super-suits era, with a time of 3:10.34, finishing 0.28 seconds after the gold-medal-winning team.

[43] He registered his 59th and 60th sub-22 second long course 50-meter freestyles on 17 June 2018 at the Mare Nostrum Tour, passing country-mate and Olympic gold medalist Cesar Cielo as the most frequent 21-second sprinter in history.

[44] In September 2018, Fratus underwent surgery to correct a partial rupture of the subscapularis tendon of the shoulder, missing out on important competitions.

[49] At the 2019 Pan American Games held in Lima, Peru, Fratus won the gold medal in the Men's 50 metre freestyle, defeating Nathan Adrian, with a time of 21.61.