Cielo trained at the Esporte Clube Pinheiros until 2009.
He worked with Coach Albertinho at Pinheiros from 15 to 22 years old.
In May 2009, Cielo broke the South American record in the 50-metre butterfly, but the record lasted only a few minutes.
Cielo achieved a time of 23.49 seconds competing for the Maria Lenk Trophy but, in the next heat, Guilherme Roth swam a time of 23.46 seconds and established a new continental record.
In the final, Cielo regained the record with a time of 23.42 seconds.
In the same competition, he broke the South American record for the 100-metre freestyle, with a time of 47.60 seconds.

At the 2009 US World Championship Trials in Indianapolis, Cielo swam a time of 21.14 seconds in the B final of the 50-metre freestyle (the A final was limited to Americans).
His time was the fastest time among all competitors, and set a new Americas record.
It was also the 2nd fastest time in the event's history, 0.20 seconds behind Frédérick Bousquet's world record.

At the 2009 World Aquatics Championships in Rome, Cielo led the Brazilian 4×100-metre freestyle relay team to a 4th-place finish, along with Nicolas Oliveira, Guilherme Roth and Fernando Silva.
In this event, he opened with a time of 47.39 seconds in the heats (a South American and Championship record) and a time of 47.09 seconds in the final, only 0.04 seconds from beating Eamon Sullivan's world record, and earning the second fastest time in the history of the 100-metre freestyle.
In the 100-metre freestyle final, the Brazilian swimmer won the gold medal, defeating the Olympic champion Alain Bernard and breaking the world record with a time of 46.91 seconds, entering into the select pantheon of swimmers who won an Olympic gold, a World Championships gold, and a World Record.
In the 50-metre freestyle final, Cielo defeated world record holder Frédérick Bousquet and won the gold medal with a time of 21.08 seconds, beating the competition record and the Americas record.
Cielo became the third swimmer to achieve this feat in a single World Championship, after Anthony Ervin and Alexander Popov.
Popov, Ervin, and Cielo each won Olympic and World Championship gold medals in the 50-metre freestyle in succession.
When he finished the 4×100-metre medley, a contest where the first four relays in the race beat the US world record from the 2008 Summer Olympics, Cielo led Brazil to fourth place, along with Guilherme Guido, Henrique Barbosa and Gabriel Mangabeira, very close to winning the event's bronze and silver medals.
His two gold medals at the World Championships led Brazil to the best performance in the history of the competition.

Cielo became the sixth Brazilian to achieve a world record in the long course, after Maria Lenk, Manuel dos Santos, José Fiolo, Ricardo Prado and Felipe França.
It was the 13th world record made by a Brazilian swimmer.

On 18 December 2009, in São Paulo, Cielo broke the world record in the 50-metre freestyle, with a time of 20.91 seconds, at a championship hosted at Esporte Clube Pinheiros, his club in Brazil and where he had trained since 2003.
It was the last official event allowing the use of super-suits in Brazil.
A day later, he established the best time in the world, 1:26.12, in the 4×50-metre freestyle.
The time was not considered a world record because this event is not part of the Olympics and World Championships.
Cielo trained at the Esporte Clube Pinheiros until 2009. He worked with Coach Albertinho at Pinheiros from 15 to 22 years old. In May 2009, Cielo broke the South American record in the 50-metre butterfly, but the record lasted only a few minutes. Cielo achieved a time of 23.49 seconds competing for the Maria Lenk Trophy but, in the next heat, Guilherme Roth swam a time of 23.46 seconds and established a new continental record. In the final, Cielo regained the record with a time of 23.42 seconds. In the same competition, he broke the South American record for the 100-metre freestyle, with a time of 47.60 seconds.
At the 2009 US World Championship Trials in Indianapolis, Cielo swam a time of 21.14 seconds in the B final of the 50-metre freestyle (the A final was limited to Americans). His time was the fastest time among all competitors, and set a new Americas record. It was also the 2nd fastest time in the event's history, 0.20 seconds behind Frédérick Bousquet's world record.
At the 2009 World Aquatics Championships in Rome, Cielo led the Brazilian 4×100-metre freestyle relay team to a 4th-place finish, along with Nicolas Oliveira, Guilherme Roth and Fernando Silva. In this event, he opened with a time of 47.39 seconds in the heats (a South American and Championship record) and a time of 47.09 seconds in the final, only 0.04 seconds from beating Eamon Sullivan's world record, and earning the second fastest time in the history of the 100-metre freestyle. In the 100-metre freestyle final, the Brazilian swimmer won the gold medal, defeating the Olympic champion Alain Bernard and breaking the world record with a time of 46.91 seconds, entering into the select pantheon of swimmers who won an Olympic gold, a World Championships gold, and a World Record. In the 50-metre freestyle final, Cielo defeated world record holder Frédérick Bousquet and won the gold medal with a time of 21.08 seconds, beating the competition record and the Americas record. Cielo became the third swimmer to achieve this feat in a single World Championship, after Anthony Ervin and Alexander Popov. Popov, Ervin, and Cielo each won Olympic and World Championship gold medals in the 50-metre freestyle in succession. When he finished the 4×100-metre medley, a contest where the first four relays in the race beat the US world record from the 2008 Summer Olympics, Cielo led Brazil to fourth place, along with Guilherme Guido, Henrique Barbosa and Gabriel Mangabeira, very close to winning the event's bronze and silver medals. His two gold medals at the World Championships led Brazil to the best performance in the history of the competition.
Cielo became the sixth Brazilian to achieve a world record in the long course, after Maria Lenk, Manuel dos Santos, José Fiolo, Ricardo Prado and Felipe França. It was the 13th world record made by a Brazilian swimmer.
On 18 December 2009, in São Paulo, Cielo broke the world record in the 50-metre freestyle, with a time of 20.91 seconds, at a championship hosted at Esporte Clube Pinheiros, his club in Brazil and where he had trained since 2003. It was the last official event allowing the use of super-suits in Brazil. A day later, he established the best time in the world, 1:26.12, in the 4×50-metre freestyle. The time was not considered a world record because this event is not part of the Olympics and World Championships.
César Cielo