Sebastián Decoud was a very promising player on the Junior Tour and achieved his best ranking in 1999 with 32nd place, but did not play in any of the Junior Grand Slams. After that he played the first two years of his professional career (2000 and 2001) exclusively tournaments on the third-class ITF Future Tour . There he was able to win a total of 12 single and 15 double titles. His first appearance on the ATP Challenger Tour was in 2002 in Quito . In the following years he appeared sporadically on the Challenger Tour, but did not play tournaments there regularly until 2006. One year later he won the singles tournament in Constanța and doubles the tournaments in Graz and Geneva . In 2008, Decoud qualified in Costa do Sauípe for the individual main field and won his opening match against the ranked seventh José Acasuso . In the second round he was eliminated against Nicolás Lapentti . At the French Open in the same year he failed in the qualifying round, but came to the main draw of the tournament as a lucky loser due to numerous injuries-related cancellations . In the first round he won in five sets against Olivier Rochus , who was then number 66 in the world. In the second round he met Nicolás Almagro , to whom he clearly had to admit defeat in three sets. On the Challenger Tour he won his second singles title in Almaty in September and his third doubles title in Guayaquil in November alongside Santiago Giraldo . He finished the second best season of his career for the first time in the top 200 of the world rankings in 180th place.
2009 started quite successfully for Decoud. He moved into the main draw in Viña del Mar again as a lucky loser . There he surprisingly managed two wins in the first two rounds against the then top 100 players Fabio Fognini and Agustín Calleri . In the quarterfinals, the later finalist José Acasuso was finally a size too big for Decoud and he was eliminated. Two months later he made it to the final at the Challenger in Rome . There he met Simon Greul , whom he defeated in two sets and thus won his third individual title. In doubles, his challenger season was even more successful, he was able to win both the tournament in Rijeka with Miguel Ángel López Jaén and the tournament in Medellín with Eduardo Schwank . This year Decoud achieved his best placings in singles with a 132nd place in the world rankings and in doubles with a 177th place. In the following year, Decoud played mainly on the Challenger Tour, but never got past the semi-finals and could not qualify for any tournament on the ATP World Tour . It wasn't until 2011 that he played another match on the World Tour. In Newport he met Nicolas Mahut in the first round . However, he had no chance in this match and clearly lost with 1: 6, 0: 6. His appearance at the Challenger tournament in Quito was more successful . Without losing a set, he played his way to the final and met Daniel Muñoz de La Nava there . Decoud continued his series without losing a set and also won the duel in two sets. His last appearance on the professional tour was in 2012 at the Challenger in Buenos Aires , where he was eliminated in the first round.