Athletic Bilbao scored their biggest win of the season as they pulled off a huge 1-0 upset win against league-leading Real Madrid on Saturday at San Mames in Bilbao.

Heading into the match as a heavy underdog going up against the star-studded Real Madrid squad, Athletic Bilbao got a huge morale booster off a goal from Aritz Aduriz, which turned out to be the lone and game-winning goal of the match.

Aduriz took a perfect cross from veteran midfielder Mikel Rico and nailed in a header from the center to the top right corner of the goal in the 26th minute of the match, catching multi-titled goalkeeper Iker Casillas and the entire Real Madrid squad by surprise.

Cristiano Ronaldo had a chance to score an equalizer before the first half ended. Toni Kroos took a corner kick conceded by Rico, but Ronaldo's header missed to the right of the box.

Coming off the break, Ronaldo had two chances to give Real Madrid their first goal. However, his right-footed shot in the 50th minute was blocked, while another one missed to right.

Isco also had a scoring chance for Real Madrid in the 58th minute, but his attempt was saved in the bottom left corner by Athletic Bilbao goalie Gorka Iraizoz, who also denied Karim Benzema's strike in the 61st minute.

With their backs against the wall, Gareth Bale took a 40-yard left-footed strike in the 83rd minute, but his attempt hit the left post, while Jese's shot was also blocked. Lucas Silva had a desperation kick in the 90th minute, but it went high before the final whistle sounded to the delight of more than 52,000 fans in San Mames.

With the loss, Real Madrid are now in danger of losing the top spot in the Spanish La Liga table. They are still on top at this point with 61 points in 26 games, but FC Barcelona could take their spot with a win on Sunday. The Lionel Messi-led FC Barcelona will take on Rayo Vallecano at Camp Nou.