Sergio Fernández first appeared at the Spanish youth championships in 2011. A year later he qualified for the U20 World Championships in Barcelona, where he took on the 400m hurdles and reached the semi-finals. In 2014 he won bronze over 400 m hurdles at the U23 Mediterranean Games. Over this distance he also competed at the European Championships in Zurich, and thus in his first international championship race among adults. There, too, he managed to qualify for the semifinals, in which he was eliminated as fifth in his run in 50.89 s.
In 2015 Fernández competed at the U23 European Championships in Tallinn. He managed to get into the final, in which, however, he finished last. In 2016 he was able to record his greatest sporting success to date with the vice European title at the European Championships in Amsterdam. At the end of the championships he also appeared in the quartet of the Spanish 4 x 400 meter relay, with which he missed the finals as fifth in the intermediate run. In August he also took on the 400 m hurdles at the Olympic Games in Rio. He survived the run in fifth place in his run. In the semifinals, he was not able to qualify for the final in third place due to a slower run, but he still ran a new Spanish record in this discipline in 48.87 s, which meant 11th place in the end.
A year later Fernández failed at the World Championships in London in the run-up. At the European Championships 2018 in Berlin, he ran to seventh place in the final with an improved time compared to the World Championships. In October 2019, he missed the semi-finals at the World Championships in Doha in sixth place in his preliminary run.
Spain
Zurich
Tallinn
Amsterdam
Rio de Janeiro
London
Berlin
Doha