In May 2004, del Potro won his first senior match, at the age of 15, at the ITF Circuit event in Buenos Aires by defeating Matias Niemiz. He then went on to lose in three sets to Sebastián Decoud in the second round. His next victory came over five months later against the Chilean Álvaro Loyola in a tournament in Antofagasta. Later that year, del Potro reached the quarterfinals of the ITF Circuit event in Campinas, Brazil; recording victories over Henrique Mello and Alessandro Camarco. Del Potro won two more matches before the end of the year and saw his world ranking rise from no. 1441 in August to no. 1077 in November. He also reached the finals in the Argentina Cup and Campionati Internazionali D'Italia Junior tournaments.