Grand Slam debut in the 2004 French Open as a qualifier, losing in the second round to eventual finalist and compatriot Guillermo Coria. He also reached the quarterfinals of the 2004 Swedish Open, losing to Chilean Fernando González. He got his best result of the year reaching his first semifinal in 2004 Idea Prokom Open in Sopot (l. to José Acasuso) and 2004 Campionati Internazionali di Sicilia in Paloma (l. to Filippo Volandri). He also made his 2004 US Open debut, but lost to compatriot Gastón Gaudio in the first round. He ended the year in the top 100 for the first time, ranked at no. 73 in singles. In 2005, Monaco made his 2005 Australian Open debut, losing in the first round to American Mardy Fish. He reached the second round of the 2005 Pacific Life Open and the quarterfinals in 2005 BMW Open, both times losing to compatriot David Nalbandian. He reached his first ATP finals in Casablanca at the 2005 Grand Prix Hassan II, but lost to compatriot Mariano Puerta. He reached the second round of 2005 Internazionali BNL d'Italia and the 2005 Hamburg Masters, losing to Guillermo Cañas and Jiří Novák, respectively. He lost in the first round of the 2005 French Open to Sébastien Grosjean. He made his Wimbledon debut, losing to Novak Djokovic. However, he got his first top-20 win, defeating Spaniard David Ferrer, but lost the next round to Tomáš Zíb in the 2005 Mercedes Cup Open. He reached the quarterfinals in the Vietnam Open, losing to Radek Štěpánek. He ended the year 12 places lower than the previous year, at no. 85. In 2006, Monaco recorded three first-round exits in the first two months in Buenos Aires, Santiago, Chile, and Adelaide and second-round exits at the Australian Open and the 2006 Medibank International. However, he rebounded by reaching the semifinals in the Brasil Open, losing to Nicolás Massú of Chile. Following the semifinal appearance, he lost four straight matches in Acapulco, the 2006 NASDAQ-100 Open, the 2006 U.S. Men's Clay Court Championships, and Morocco. He also lost in the first round of the 2006 Internazionali BNL d'Italia, losing to veteran French player Fabrice Santoro, and lost in the second round of the 2006 Hamburg Masters to Robin Söderling. Following these losses, Monaco achieved great results in the next four weeks, reaching the quarterfinals in Poertschach (l. to Luis Horna), the third round