Born in Santander, Cantabria, Sierra played with amateur clubs in his native region until 1998 when he signed with Racing de Santander, first being assigned to the B team also in the lower leagues.
Sierra was born in Santander, Cantabria, and started playing with amateur teams in his hometown until he joined Racing de Santander in 1998, initially being sent to their reserve team that competed in lower-level leagues.