Match ends, Athletic Club 1, Real Madrid 1.
Substitute Isco rescued a point for Real Madrid as their winning start to the season came to an end against Athletic Bilbao.
Iker Muniain had put Eduardo Berizzo's Athletic ahead on 32 minutes after Real failed to clear their lines.
But, less than three minutes after coming on, Isco converted Gareth Bale's cross to earn Los Blancos a point.
Earlier, Barcelona came from behind to maintain their perfect start to the season, defeating Real Sociedad 2-1.
Real had won their first three games in La Liga, scoring four goals in each of their previous two games, and they started quickly at the San Mames Stadium, Luka Modric seeing an effort blocked in the opening minute.
But Athletic, playing just their second match of the season, settled into the game and caused the visitors problems, with both Markel Susaeta and Yuri Berchiche registering early attempts on former Chelsea goalkeeper Thibaut Courtois' goal.
Despite not playing a competitive fixture in almost three weeks, following the postponement of their match against Rayo Vallecano before the international break, the hosts were the better side in the first half and limited Julen Lopetegui's side to long-range attempts.
Athletic's 21-year-old goalkeeper Unai Simon produced several good saves when called upon, denying both Marco Asensio and Modric - but he was powerless to prevent Isco's glancing header on 63 minutes.
A resurgent Real could have been ahead soon after, but the impressive Simon denied Sergio Ramos from close range.
Neither side was able to force a winner in the closing stages, however, as Real - who remain second in the table - lost early ground on champions Barcelona.
Next Lopetegui's side begin the defence of their Champions League crown at home to Roma on Wednesday (20:00 BST), before returning to league duties against Espanyol on Saturday (19:45 BST).
Match ends, Athletic Club 1, Real Madrid 1.
Second Half ends, Athletic Club 1, Real Madrid 1.
Corner, Athletic Club. Conceded by Raphael Varane.
Foul by Daniel Carvajal (Real Madrid).
Íñigo Martínez (Athletic Club) wins a free kick in the defensive half.
Sergio Ramos (Real Madrid) wins a free kick in the defensive half.
Foul by Óscar De Marcos (Athletic Club).
Attempt saved. Mikel Rico (Athletic Club) right footed shot from outside the box is saved in the bottom left corner. Assisted by Ander Capa.
Unai Simón (Athletic Club) is shown the yellow card.
Offside, Real Madrid. Sergio Ramos tries a through ball, but Isco is caught offside.
Casemiro (Real Madrid) wins a free kick in the defensive half.
Foul by Raúl García (Athletic Club).
Lucas Vázquez (Real Madrid) is shown the yellow card for a bad foul.
Foul by Lucas Vázquez (Real Madrid).
Dani García (Athletic Club) wins a free kick in the defensive half.
Sergio Ramos (Real Madrid) wins a free kick in the attacking half.
Foul by Ander Capa (Athletic Club).
Offside, Athletic Club. Íñigo Martínez tries a through ball, but Ander Capa is caught offside.
Attempt missed. Íñigo Martínez (Athletic Club) right footed shot from outside the box misses to the right.
Daniel Carvajal (Real Madrid) is shown the yellow card for a bad foul.
Foul by Daniel Carvajal (Real Madrid).
Raúl García (Athletic Club) wins a free kick in the defensive half.
Marco Asensio (Real Madrid) wins a free kick in the attacking half.
Foul by Yeray Álvarez (Athletic Club).
Attempt missed. Marcelo (Real Madrid) left footed shot from the left side of the box is too high.
Substitution, Athletic Club. Mikel Rico replaces Beñat Etxebarria.
Offside, Real Madrid. Marco Asensio tries a through ball, but Marcelo is caught offside.
Beñat Etxebarria (Athletic Club) is shown the yellow card for a bad foul.
Isco (Real Madrid) wins a free kick in the defensive half.
Foul by Beñat Etxebarria (Athletic Club).
Substitution, Athletic Club. Mikel San José replaces Iñaki Williams.
Substitution, Real Madrid. Lucas Vázquez replaces Gareth Bale.
Attempt missed. Sergio Ramos (Real Madrid) right footed shot from outside the box is too high from a direct free kick.
Yuri (Athletic Club) is shown the yellow card for a bad foul.
Isco (Real Madrid) wins a free kick in the attacking half.
Foul by Yuri (Athletic Club).
Attempt missed. Iñaki Williams (Athletic Club) left footed shot from outside the box is high and wide to the left.
Attempt blocked. Markel Susaeta (Athletic Club) right footed shot from the centre of the box is blocked.
Attempt missed. Karim Benzema (Real Madrid) right footed shot from outside the box is high and wide to the right.
Attempt saved. Marco Asensio (Real Madrid) right footed shot from the right side of the box is saved in the centre of the goal. Assisted by Sergio Ramos with a through ball.