Sergio Asenjo is considered one of the biggest goalkeeping talents of Spain and as the designated successor to Iker Casillas of Real Madrid in the national team. Sergio Asenjo began in the youth team of Real Valladolid. Only 17 years old, he was goalkeeper in Segunda División B, the 3rd Liga in Spain. In the summer of 2007, he was brought in as third goalkeeper in the first team. With this team he had at the age of 18 years, made his debut in the first division after the first two goalkeeper failed due to an injury. Meanwhile, the actual goalkeeper has played 13 season games. In January 2008, he signed a professional contract significantly improved and increased in the goalkeeper ranking at number one on.
Asenjo joined the 2008/09 season at Atlético Madrid, where he had after the departure of Grégory Coupet and Leo Franco good prospects for a starting berth. After injury- related failure due to a cruciate ligament injury but took over the also young, new to the team bumped David de Gea his hitherto undisputed regular place and displaced simultaneously Roberto to number 3 David de Gea was then goalkeeper, returned as Asenjo after his injury in the squad.
Halfway through the 2010/11 season gave Atlético Madrid Sergio Asenjo until season's end to the league rivals FC Malaga. There he was to play matches again. However Asenjo completed only five games, the reason was a renewed cruciate ligament. After the 2010/11 season he returned to Atletico, but had to be content with the role of number two behind early in the season Thibaut Courtois and before Joel.
For the 2013/14 season Asenjo joined on loan to FC Villarreal.
At the international level Sergio Asenjo could draw attention to themselves. In the U-17 European Championship in 2006 he won the bronze medal with Spain, 2007 with the Spanish U-19 even the title of European Champion. In the semifinals, he held two balls in the penalty shootout against France in the final he was in the 1-0 victory over Greece tight at the back.
- UEFA Europa League: 2010, 2012
- Third place in the U-17 European Championship: 2006
- U-19 European Championship: 2007