In mid-2004, Massú parted ways with Argentine coach Gabriel Markus, whom he replaced with Chilean Patricio Rodríguez. In July 2004, Massú won his fourth ATP title in Kitzbühel and then went on to win two gold medals at the 2004 Olympics (see below). Thanks to his outstanding performance at the Olympics, he reached his career-high ATP singles ranking of world No. 9. In November, he underwent groin surgery and therefore entered the 2005 season off top form. He ended an unremarkable 2005 with a six-match losing streak, although ironically 2005 also saw his best performance at a Grand Slam tournament as he reached the fourth round of the US Open, losing to Guillermo Coria.