Del Potro turned professional after the Italy F17 event in Bassano, and in his first professional tournament, the Lines Trophy in Reggio Emilia, he reached the semifinals, where he lost to countryman Martín Vassallo Argüello in three sets. Two tournaments later, he reached the final of the Credicard Citi MasterCard Tennis Cup in Campos do Jordão, Brazil, where he lost to André Sá in straight sets. After turning 17, he won the Montevideo Challenger by defeating Boris Pašanski in the final in three sets. That same year, he failed in his first attempt to qualify for his first Grand Slam, at the US Open, losing in the first round to Paraguayan Ramón Delgado. Throughout 2005, del Potro jumped over 900 positions to finish with a world ranking of no. 157, largely due to winning three Futures tournaments. He was the youngest player to finish in the year-end top 200.