2001–2004 His first participation in the World Championships was in the
2001 World Aquatics Championships in Fukuoka, where he finished 30th in the 50-metre freestyle. On 17 November, he broke the short-course South American record of the 50-metre butterfly, with a time of 23.82 seconds. At the
2002 FINA World Swimming Championships (25 m) in
Moscow, Nicholas finished 23rd in the 50-metre freestyle and 13th in the 50-metre butterfly. He swam at the
2002 Pan Pacific Swimming Championships, where he finished 9th in the 50-metre freestyle. Participating in the
2003 World Aquatics Championships in
Barcelona, Nicholas finished 40th in the 50-metre butterfly. On 10 September 2004, Santos broke the short-course South American record of the 50-metre freestyle with a time of 21.32 seconds, coming close to beat the
World Record that belonged to
Frédérick Bousquet with 21.10 seconds. At the
2004 FINA World Swimming Championships (25 m) in
Indianapolis, Nicholas Santos won the silver medal in the 4 × 100-metre freestyle and bronze in the 50-metre freestyle. He also participated in the 50-metre butterfly, where he was disqualified, and the 100-metre freestyle, where he was in the finals, placing 8th.
2005–2008 at
2007 Pan American Games At the
2005 Summer Universiade, Nicholas won a bronze medal in the 50-metre freestyle and in the 50-metre butterfly. Santos swam at the
2006 Pan Pacific Swimming Championships, where he finished 10th in the 50-metre freestyle. At the
2007 Pan American Games in
Rio de Janeiro, he became champion in 4 × 100-metre freestyle, with a new Pan-American record made by the Brazilian team: 3:15.90, and was also a silver medalist in the 50-metre freestyle, with a time of 22.18 seconds. Nicholas competed in the
2007 Summer Universiade, where he won a gold medal in the 50-metre freestyle and a silver medal in the 50-metre butterfly. He broke the competition record in the 50-metre freestyle, with a time of 22.12 seconds.
2008 Summer Olympics Nicholas participated in the
2008 Summer Olympics in the 50-metre freestyle, where he reached the semi-finals, finishing in 16th position.
2009–2012 In 10 May 2009, he broke the South American record in the 4 × 100-metre freestyle, with a time of 3:14.45, along with
Nicolas Oliveira,
César Cielo, and
Fernando Silva. At the
2009 World Aquatics Championships in
Rome, he qualified for the 50-metre butterfly finals, finishing in 5th place. He also reached the semi-finals of the 50-metre freestyle, with a time of 21.69 seconds, and finished in 10th place. On 2 September 2009, at the end of the
José Finkel Trophy, he won with a time of 21.55 seconds, surpassing his personal best. In 4 September 2009, he broke the Americas record for the 50-metre butterfly, with a time of 22.87 seconds. This record was broken by
César Cielo just three years later, with a time of 22.76 seconds. In 15 November 2009, in short pool (25 m), he completed the 50-metre freestyle in 20.90 seconds and became the first South American to swim the distance under 21 seconds. He shared with
César Cielo the short-course South American record in the 50-metre freestyle: 21.32 seconds. He established the time in 2004 and Cielo in 2008. He was champion of the
2009 FINA Swimming World Cup at the Singapore step in November 2009, beating the Olympic Champion
Roland Schoeman in the finals of both the 50-metre butterfly and the 50-metre freestyle. At this World Cup, he broke the South American record of the 50-metre butterfly with a time of 22.17 seconds in Berlin and 22.16 seconds in Singapore and the South American record of the 50-metre freestyle with a time of 20.74 seconds in Berlin. In August 2010, at the
2010 Pan Pacific Swimming Championships in
Irvine, California, United States, Nicholas earned the silver medal in 50-metre butterfly. He also finished 13th in the 50-metre freestyle. In December 2010, at the
2010 FINA World Swimming Championships (25 m) in
Dubai, Nicholas Santos, along with
César Cielo,
Marcelo Chierighini, and
Nicolas Oliveira, won the bronze medal in the 4 × 100-metre freestyle with a time of 3:05.74, South American record, leaving behind the American team. Nicholas also got the 13th place in the 50-metre freestyle and 4th place in the 50-metre butterfly. At the
2011 Pan American Games in
Guadalajara, Nicholas won the gold medal in the 4 × 100-metre freestyle. In April 2012, he improved his personal best in 50-metre butterfly to 22.79 seconds. At the
2013 World Aquatics Championships in
Barcelona, Santos qualified to the
50-metre butterfly finals in the first place, with a time of 22.81 seconds. In the finals, his time worsened to 23.21 seconds, finishing in 4th place. In the 4 × 100-metre medley, he finished 12th, along with
Leonardo de Deus,
Felipe Lima, and
Marcelo Chierighini. At the
2013 FINA Swimming World Cup in Beijing, China, Santos broke the short-course South American record in the 50-metre butterfly, with a time of 22.13. At the
2014 Pan Pacific Swimming Championships in
Gold Coast, Queensland, Australia, he finished 8th in the 100-metre butterfly, and 16th in the 50-metre freestyle. At the
2014 FINA World Swimming Championships (25 m) in
Doha,
Qatar, Santos won two gold medals on the same day. At 4 December, in the
Men's 4 × 50 metre medley relay, formed by Santos,
César Cielo,
Felipe França Silva and
Guilherme Guido, considered the "Dream Team" by Cielo (formed only by medalists or world champions in their respective individual events), Brazil won the gold shattering the world record with a time of 1:30.51. At the same day, he also won the gold medal in the
4 × 50 metre mixed medley relay, along with
Felipe França Silva,
Etiene Medeiros and
Larissa Oliveira, breaking the South American record with a time of 1:37.26, only 0.09 seconds from beating USA's world record (1:37.17). Santos was also trying to defend his title in the
Men's 50 metre butterfly, obtained in Istanbul 2012. In the final, he faced
Chad le Clos, the world's best butterfly swimmer at the time, World and Olympic champion. Santos broke the Americas record, with a time of 22.08, but lost the gold to the South African, who broke the Championship record, with a time of 21.95. Santos also swam the
Men's 100 metre butterfly, finishing in 14th place. At the
2015 Pan American Games in
Toronto, Ontario, Canada, Santos finished 10th in the 50-metre freestyle. At the
2015 World Aquatics Championships in
Kazan, Santos, 35 years old, won one of the most important medals in his career, the silver medal in the
Men's 50 metre butterfly. He became the oldest medalist in the history of the World Championships – aged 35 years and 171 days old, broke the record of Mark Warnecke, gold medalist of the 50 breaststroke at Montreal 2005, which took the podium at 35 years and 162 days old.
2016–2022 At the
2016 FINA World Swimming Championships (25 m) in
Windsor, Ontario, Canada, he won a silver medal at the
4 × 50 metre mixed medley relay, along with
Etiene Medeiros,
Larissa Oliveira and
Felipe Lima. He also finished 9th in the
Men's 50 metre butterfly and 32nd in the
Men's 50 metre freestyle. At the age of 37, at the Maria Lenk Trophy held in May 2017, he was able to beat the Americas record in the 50m butterfly, with a time of 22.61, being close to the world record of 22.43 did by Rafa Muñoz, at the super-suits era. At the
2017 World Aquatics Championships in
Budapest, in the
Men's 50 metre butterfly, he won again the silver medal, with a time of 22.79. He broke a record that was his own: the oldest athlete in history to win a medal in the World Championships. At the
2018 FINA Swimming World Cup in
Budapest, he broke the
short-course World Record in the 50-metre butterfly, with a time of 21.75. At the
2018 FINA World Swimming Championships (25 m) in
Hangzhou,
China, in the
Men's 50 metre butterfly, Santos won the gold medal with a time of 21.81, a new Championship record. He also won a bronze medal in the
Men's 4 × 50 metre medley relay, along with
Guilherme Guido,
Felipe Lima and
César Cielo. In the
Men's 4 × 100 metre medley relay, he finished 4th, and also finished 9th in the
Mixed 4 × 50 metre medley relay. At the age of 39, at the FINA Champions Swim Series held in May 2019 in Budapest, he was able to beat the Americas record in the 50m butterfly, with a time of 22.60. That was the best time of the year. Santos went to the
2019 World Aquatics Championships in
Gwangju,
South Korea thanks to an invitation from FINA, due to the fact that the CBDA, the Brazilian Swimming Confederation, only summoned to this Championship the swimmers of Olympic events. There he won the bronze medal in the
Men's 50 metre butterfly at the age of 39, "updating" his record for the oldest swimmer in the world to win a World Championship medal. At the
2021 FINA World Swimming Championships (25 m) in
Abu Dhabi,
United Arab Emirates, in the
Men's 50 metre butterfly, Santos won the gold medal with a time of 21.93. At almost 42 years old, he extended his own record for the oldest swimmer ever to win a world championship title. On 14 December 2022, at the
2022 FINA World Swimming Championships (25 m), in Melbourne, Australia, in the
50-metre butterfly, Santos won his fourth world championship title with a time of 21.78, breaking the Championship Record, 0.03s off his own world record of 21.75. At the age of 42, he broke his own record for being the oldest swimmer to win a world title and being the only and first swimmer in the world to be a four-time world champion in the 50m butterfly event. After the gold medal ceremony, he announced his retirement from the sport. ==Records==