Athletic Bilbao's top four finish ambitions were dented in a narrow 1-0 defeat by Valencia on Saturday in LaLiga.
Valencia triumphed over their third-place visitors in an even battle, with Hugo Duro's header splitting the sides at their Mestalla stadium.
Ernesto Valverde's Athletic were unbeaten in 14 matches across all competitions and had won their previous six.
However they came up against another in-form side in Ruben Baraja's sturdy Valencia, who ground out their fourth consecutive La Liga victory to move seventh.
"The worst thing is we had control of the game, we got up there well but didn't make many dangerous chances," said Valverde.
Sergi Canos went off injured for the hosts early on, while Valencia goalkeeper Giorgi Mamardashvili made two saves from Nico Williams.
The winger's brother Inaki Williams is away at the Africa Cup of Nations with Ghana and Athletic missed his dynamism in attack.
Oihan Sancet fired narrowly off-target for Athletic after the break, before Duro missed a huge chance from close range for Valencia.
However two minutes later he broke the deadlock by nodding Jose Gaya's looping cross beyond the reach of Unai Simon in the 61st minute.
Mamardashvili made a superb save to thwart Unai Gomez as the Basques chased the leveller, but Valencia held firm as Athletic suffered only their fourth top flight defeat.
After battling relegation last season and only securing survival on the final day, Valencia are enjoying a good campaign and their players celebrated joyfully with supporters at full time.
"It's a moment to enjoy, the fans, the atmosphere at the stadium, how the players have worked so hard... it's a fantastic day," said Baraja.
"I'm thinking I will enjoy these moments and let them stay in my retinas."
Barcelona, fourth, can now overtake Athletic if they avoid defeat at Real Betis on Sunday.
League leaders Girona host Sevilla and Real Madrid, second, face bottom side Almeria.
MENDEZ APOLOGY
Brais Mendez's long-range strike earned Real Sociedad a 1-0 win at Celta Vigo.
Victory in Galicia ensured Imanol Alguacil's side, sixth, kept pressure on the teams above them.
Playmaker Mendez sent the visitors ahead with a superb strike from around 30 yards out which flew home past the despairing Vicente Guaita after 11 minutes.
The former Celta Vigo midfielder raised his hands to apologise for his goal instead of celebrating but the damage was done, having scored for the first time since October.
Guaita denied Umar Sadiq, who should have doubled the visitors' lead before they had to take off defender Aihen Munoz with a worrying knee injury.
Mendez hit the post with a low free-kick bent around the wall early in the second half as La Real dominated, with Robin Le Normand also inches away from scoring.
Real Sociedad goalkeeper Alex Remiro saved Celta's best effort, a clever flicked effort by Luca de la Torre.
Rafa Benitez's side increased the pressure in the final stages but could not find a way through, meaning they are provisionally 16th, two points above the relegation zone.
Celta have the chance to gain revenge quickly, as they face Real Sociedad again at Balaidos on Tuesday in the Copa del Rey quarter-finals.
Earlier on Saturday Las Palmas, eighth, earned a solid 2-0 win at Rayo Vallecano, while Real Mallorca struck late to hold Villarreal to a 1-1 draw.