Real Madrid made Athletic Club pay for missed chances as Karim Benzema's goal secured a 1-0 win at the Santiago Bernabeu to move Carlo Ancelotti's side seven points clear at the top of LaLiga.
Benzema struck the winning goal in the 40th minute against the run of play to take his tally for the season to 17 in all competitions.
The victory moves Madrid seven clear of Atletico, who have a game in hand, after 15 matches.
Athletic remain eighth and are now without a win in six LaLiga matches, although four of those games have ended in draws.
Marcelino's side were the better side at the Santiago Bernabeu on Wednesday but could not take their chances.
Iñaki Williams missed two good chances in the first half and Raul Garcia saw a header saved by Thibaut Courtois.
The Belgian goalkeeper continued his incredible recent form in the second half to keep Athletic out as Madrid held on for a big three points.