In 2008, Monaco started the year with a semifinal appearance at the 2008 Heineken Open, losing to German Philipp Kohlschreiber.
He also reached the third round of the 2008 Australian Open, the furthest he has reached so far, losing to Tomáš Berdych.
In his first tournament after the Australian Open, he reached the final of the 2008 Movistar Open.
He was scheduled to play Fernando González in the final, but Gonzalez was granted a walkover due to a left ankle injury sustained by Monaco during the doubles final.
Despite this, his performance during the week elevated Monaco to a career-high ranking of world no.
14.

At the 2008 Copa Telmex as the defending champion, Monaco lost in the first round.
Shortly afterwards, he lost in the second round of the 2008 Abierto Mexicano Telcel in Acapulco.
He also reached the third round of both the 2008 Pacific Life Open in Indian Wells, losing to Guillermo Cañas, and the 2008 Sony Ericsson Open in Miami, losing to Mario Ančić.
He represented Argentina in the Davis Cup quarterfinal clash against Sweden and won his only match against Thomas Johansson.

He reached the quarterfinals in Valencia and the second round of the 2008 Monte Carlo Masters and 2008 Rome Masters losing to Nicolás Almagro and Igor Andreev, respectively.
He also reached the third round of the 2008 Hamburg Masters, losing to Andreas Seppi and the final, of 2008 Hypo Group Tennis International losing to top seed Nikolay Davydenko.
He reached the semifinals of the 2008 Orange Warsaw Open, losing to Tommy Robredo.
At the Beijing Olympics, Monaco lost to Marin Čilić in three tight sets 4–6, 7–6, 3–6, followed by successive defeats at the 2008 Pilot Pen Tennis and the 2008 US Open.
He fell in the first round of the 2008 Mutua Madrileña Masters Madrid, losing to Radek Štěpánek, and the 2008 Grand Prix de Tennis de Lyon to Gilles Simon.
He reached the second round of both the 2008 Stockholm Open and the 2008 BNP Paribas Masters after qualifying for both tournaments.
Monaco ended the year ranked no.
46 in the world, 23 places lower than his year-end ranking in 2007.

In doubles, Monaco started the year with a win in the 2008 Heineken Open partnering Luis Horna.
He reached the semifinals of 2008 US Open, losing to Lukáš Dlouhý and Leander Paes.
He also reached the final of the 2008 Movistar Open and won the 2008 Open de Tenis Comunidad Valenciana, all partnering fellow Argentine Máximo González.
In 2008, Monaco started the year with a semifinal appearance at the 2008 Heineken Open, losing to German Philipp Kohlschreiber. He also reached the third round of the 2008 Australian Open, the furthest he has reached so far, losing to Tomáš Berdych. In his first tournament after the Australian Open, he reached the final of the 2008 Movistar Open. He was scheduled to play Fernando González in the final, but Gonzalez was granted a walkover due to a left ankle injury sustained by Monaco during the doubles final. Despite this, his performance during the week elevated Monaco to a career-high ranking of world no. 14.
At the 2008 Copa Telmex as the defending champion, Monaco lost in the first round. Shortly afterwards, he lost in the second round of the 2008 Abierto Mexicano Telcel in Acapulco. He also reached the third round of both the 2008 Pacific Life Open in Indian Wells, losing to Guillermo Cañas, and the 2008 Sony Ericsson Open in Miami, losing to Mario Ančić. He represented Argentina in the Davis Cup quarterfinal clash against Sweden and won his only match against Thomas Johansson.
He reached the quarterfinals in Valencia and the second round of the 2008 Monte Carlo Masters and 2008 Rome Masters losing to Nicolás Almagro and Igor Andreev, respectively. He also reached the third round of the 2008 Hamburg Masters, losing to Andreas Seppi and the final, of 2008 Hypo Group Tennis International losing to top seed Nikolay Davydenko. He reached the semifinals of the 2008 Orange Warsaw Open, losing to Tommy Robredo. At the Beijing Olympics, Monaco lost to Marin Čilić in three tight sets 4–6, 7–6, 3–6, followed by successive defeats at the 2008 Pilot Pen Tennis and the 2008 US Open. He fell in the first round of the 2008 Mutua Madrileña Masters Madrid, losing to Radek Štěpánek, and the 2008 Grand Prix de Tennis de Lyon to Gilles Simon. He reached the second round of both the 2008 Stockholm Open and the 2008 BNP Paribas Masters after qualifying for both tournaments. Monaco ended the year ranked no. 46 in the world, 23 places lower than his year-end ranking in 2007.
In doubles, Monaco started the year with a win in the 2008 Heineken Open partnering Luis Horna. He reached the semifinals of 2008 US Open, losing to Lukáš Dlouhý and Leander Paes. He also reached the final of the 2008 Movistar Open and won the 2008 Open de Tenis Comunidad Valenciana, all partnering fellow Argentine Máximo González.
Juan Mónaco