 After starting out at local Real Oviedo, making his senior debut aged 17 in the second division in a relegation-ending season, he finished his football development at Real Madrid, having joined in 2003.Over two seasons in the second level – of the five in total he spent with the team– Sergio made a further seven league appearances with the reserves.He was also called up to the first team for a UEFA Champions League match against FC Dynamo Kyiv in December 2006 (the group stage's last), but remained an unused substitute; he earned another call that month, in La Liga against Sevilla FC, although he ultimately was cut from the final squad of 18.