Real Madrid fell to a 2-0 defeat at Real Sociedad Tuesday, leaving the champions 14 points behind LaLiga leaders Barcelona with five games remaining.
Former Real winger Takefusa Kubo and Ander Barrenetxea netted for the hosts, who tightened their grip on fourth spot and Champions League qualification.
Real defender Dani Carvajal was sent off after an hour for a second yellow card to make matters worse for Los Blancos.
Barca beat Osasuna 1-0 earlier Tuesday to expand the gap on Real and Carlo Ancelotti's side could not respond.
The Italian played a makeshift side because of injuries and suspensions, and he also left striker Karim Benzema behind in the Madrid capital ahead of Saturday's Copa del Rey final clash with Osasuna.
Alex Remiro denied Real defender Eder Militao in the opening stages while Martin Zubimendi struck the crossbar at the other end from close range.
Kubo struck at the start of the second half for the hosts after Militao gave the ball away in a dangerous area.
Carvajal was dismissed for a foul on Sociedad forward Mikel Oyarzabal, giving Real an uphill struggle.
Real goalkeeper Thibaut Courtois did well to parry a deflected Barrenetxea strike, but he could not keep out the forward's next effort, surprising the Belgian at his near post.
Los Blancos' defeat means Barca just need two more points to clinch their first title since 2019, providing Real win their remaining matches.
