In 2009, Monaco started the year in 2009 Heineken Open losing to Serbian Viktor Troicki in the second round and a quarterfinal appearance in the 2009 Movistar Open.
He lost in the first round of 2009 Brasil Open, the 2009 Australian Open, and Acapulco.
He reached his first final of the year in Buenos Aires at the 2009 Copa Telmex, losing to Tommy Robredo in a tight three-setter.
In doubles, he reached the third round of the 2009 Australian Open and the final of 2009 Brasil Open, both partnering Lucas Arnold Ker
He represented Argentina in the Davis Cup, defeating Thiemo de Bakker of the Netherlands.
He also reached the second round of the 2009 Sony Ericsson Open, losing to Andy Murray after winning the first set.
He also reached the third round of the 2009 Monte-Carlo Rolex Masters, losing to German Andreas Beck, after defeating Tommy Robredo in the second round, and the 2009 Barcelona Open Banco Sabadell losing to Fernando González after defeating Marat Safin in the first round.
He reached his first Masters quarterfinal at the 2009 Internazionali BNL d'Italia, entering as a qualifier and beating Andy Murray in the second round, Marin Čilić in the third round, but losing to Fernando González.
He also reached the round of 16 at the 2009 Estoril Open and the 2009 Mutua Madrileña Madrid Open, where he faced three Spaniards losing to Fernando Verdasco after defeating Feliciano López and David Ferrer.
He also represented Argentina along with Juan Martín del Potro and Máximo González at the 2009 ARAG World Team Cup, where he won his only singles match against Serbian Janko Tipsarević.
He reached the second round of Roland Garros, losing to Jo-Wilfried Tsonga.

He lost in the first round of Wimbledon, still failing to capture his first win there, losing to Nicolás Almagro in a match that lasted almost four hours.
He represented Argentina once again in the quarterfinals of the 2009 Davis Cup against the Czech Republic, losing both his matches.
He then competed in the 2009 Swedish Open, defeating eighth seed Máximo González and dominating in his next three matches against Victor Crivoi, top seed Fernando Verdasco, and third seed and defending champion Tommy Robredo, before losing to Robin Söderling in the final.
In the 2009 International German Open, he defeated Mikhail Youzhny and José Acasuso.
He then lost to David Ferrer in the third round.
Monaco competed directly in the 2009 US Open without competing in any events in 2009 US Open Series.
He lost to eventual champion Juan Martín del Potro in the first round.
At the 2009 BCR Open Romania, Monaco reached his third final of the year, but once again ended as runner-up to Albert Montañés, after defeating qualifiers Júlio Silva and Pere Riba, eighth seed Pablo Cuevas, and German Simon Greul.
Because of this final appearance he ended the year with 29 wins on clay, the most wins on clay by any player.
In the 2009 Rakuten Japan Open Tennis Championships, he retired against Ernests Gulbis in the second round after defeating Guillermo García-López.
He then lost to Victor Troicki in the first round of the 2009 Shanghai ATP Masters 1000.
He lost to Joachim Johansson, after defeating Jan Hernych in the first round of the 2009 If Stockholm Open.
He competed in the 2009 Grand Prix de Tennis de Lyon as the fourth seed, but was upset by Michaël Llodra in the second round after defeating Frenchman Josselin Ouanna in the first round.
He lost in the second round of the 2009 Valencia Open 500 to Nikolay Davydenko.
He then lost to Novak Djokovic, despite leading 5–3 in the second set, after defeating Jérémy Chardy in the 2009 BNP Paribas Masters.
This was his last tournament of the year, reaching three finals; the most since winning three titles in 2007.
He ended the year at no.
30.
In 2009, Monaco started the year in 2009 Heineken Open losing to Serbian Viktor Troicki in the second round and a quarterfinal appearance in the 2009 Movistar Open. He lost in the first round of 2009 Brasil Open, the 2009 Australian Open, and Acapulco. He reached his first final of the year in Buenos Aires at the 2009 Copa Telmex, losing to Tommy Robredo in a tight three-setter. In doubles, he reached the third round of the 2009 Australian Open and the final of 2009 Brasil Open, both partnering Lucas Arnold Ker
He represented Argentina in the Davis Cup, defeating Thiemo de Bakker of the Netherlands. He also reached the second round of the 2009 Sony Ericsson Open, losing to Andy Murray after winning the first set. He also reached the third round of the 2009 Monte-Carlo Rolex Masters, losing to German Andreas Beck, after defeating Tommy Robredo in the second round, and the 2009 Barcelona Open Banco Sabadell losing to Fernando González after defeating Marat Safin in the first round. He reached his first Masters quarterfinal at the 2009 Internazionali BNL d'Italia, entering as a qualifier and beating Andy Murray in the second round, Marin Čilić in the third round, but losing to Fernando González. He also reached the round of 16 at the 2009 Estoril Open and the 2009 Mutua Madrileña Madrid Open, where he faced three Spaniards losing to Fernando Verdasco after defeating Feliciano López and David Ferrer. He also represented Argentina along with Juan Martín del Potro and Máximo González at the 2009 ARAG World Team Cup, where he won his only singles match against Serbian Janko Tipsarević. He reached the second round of Roland Garros, losing to Jo-Wilfried Tsonga.
He lost in the first round of Wimbledon, still failing to capture his first win there, losing to Nicolás Almagro in a match that lasted almost four hours. He represented Argentina once again in the quarterfinals of the 2009 Davis Cup against the Czech Republic, losing both his matches. He then competed in the 2009 Swedish Open, defeating eighth seed Máximo González and dominating in his next three matches against Victor Crivoi, top seed Fernando Verdasco, and third seed and defending champion Tommy Robredo, before losing to Robin Söderling in the final. In the 2009 International German Open, he defeated Mikhail Youzhny and José Acasuso. He then lost to David Ferrer in the third round. Monaco competed directly in the 2009 US Open without competing in any events in 2009 US Open Series. He lost to eventual champion Juan Martín del Potro in the first round. At the 2009 BCR Open Romania, Monaco reached his third final of the year, but once again ended as runner-up to Albert Montañés, after defeating qualifiers Júlio Silva and Pere Riba, eighth seed Pablo Cuevas, and German Simon Greul. Because of this final appearance he ended the year with 29 wins on clay, the most wins on clay by any player. In the 2009 Rakuten Japan Open Tennis Championships, he retired against Ernests Gulbis in the second round after defeating Guillermo García-López. He then lost to Victor Troicki in the first round of the 2009 Shanghai ATP Masters 1000. He lost to Joachim Johansson, after defeating Jan Hernych in the first round of the 2009 If Stockholm Open. He competed in the 2009 Grand Prix de Tennis de Lyon as the fourth seed, but was upset by Michaël Llodra in the second round after defeating Frenchman Josselin Ouanna in the first round. He lost in the second round of the 2009 Valencia Open 500 to Nikolay Davydenko. He then lost to Novak Djokovic, despite leading 5–3 in the second set, after defeating Jérémy Chardy in the 2009 BNP Paribas Masters. This was his last tournament of the year, reaching three finals; the most since winning three titles in 2007. He ended the year at no. 30.
Juan Mónaco