Along the seasons, he break into the first team and then he became an undisputed player with Claudio Borghi with whom won four league titles and reached a 2006 Copa Sudamericana runner-up, all of active way.

In 2009, Fierro had a more active role in the team after being chosen to replace Ibson who left the club on loan,[3] playing 24 games and scoring one goal against 2008 Série A defending champions São Paulo in a 1–1 away draw.

On 15 July 2010, was reported that Fierro joined Boca Juniors in an alleged request of Claudio Borghi to the club board.

[7] However a serious injury stopped his incorporation and on 17 August it was confirmed that he left the club for return to Rio de Janeiro with Flamengo and start his recuperation.

On 9 June, prior Group H opening game against Honduras, Chile played an unofficial friendly against New Zealand where Fierro scored the second goal of his country's 2–0 victory.