From age 12, he was trained at the Valle Dorado tennis academy, near Villa Alemana, by Leonardo Zuleta, with whom he perfected his forehand and double-handed backhand.

He later trained at the Nick Bollettieri Academy, in Florida, alongside Marcelo Ríos, and later at the High Performance Center in Barcelona, Spain.

That year, he won the prestigious juniors year-end Orange Bowl tournament and was doubles world champion, as well as No.

[12] He also claimed the boys' doubles competitions at both Wimbledon and the US Open, partnering Peru's Luis Horna[13] at the former and countryman Fernando González at the latter.

[14] In May 2000, Massú reached his first ATP tournament final, at the U.S. Clay Court Championships in Orlando, Florida, where he lost to Fernando González.

[14] Massú's first ATP title came in February 2002 in Buenos Aires, where he defeated Argentine Agustín Calleri in a three-set final, after being down match point.

At the 2003 event, Calleri took revenge and defeated him in the first round, a loss that pushed Massú out of the top 100 in singles and forced him to play Challengers once again.

In September, he made three consecutive tournament finals, including a win at a Challenger event and his third ATP title in Palermo.

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).

[14] In January 2007, Massú repeated his Viña del Mar showing of 2006, losing to Luis Horna in straight sets.

Massú had an early exit at the Viña del Mar tournament in January 2008, losing to Sergio Roitman in the first round.

He broke his losing streak at the Indian Wells Masters, beating Argentine Eduardo Schwank in three sets in the first round.

At the 2000 event's opening ceremony, he was his country's stand in flag bearer after Marcelo Ríos failed to show up.

The following day, he captured his second gold medal by defeating American Mardy Fish in five sets in the men's singles final.

To this day, Massú is the only male player in the Open Era to have won gold medals in both singles and doubles at the same Olympic Games.

[21] Massú was the coach of Dominic Thiem, 2020 US Open Men's Singles Champion and winner of the 2019 Indian Wells Masters 1000 tournament.

He has also turned around difficult matches and had a style characteristic of a clay-court specialist,[24] with strong baseline play characterized by a solid forehand and backhand.