Inigo Martinez's late header struck a blow to Atletico Madrid's La Liga title hopes as Athletic Bilbao secured a 2-1 victory on Monday (AEST).
Stefan Savic looked to have rescued a point for the leaders with his first goal in the Spanish top flight for almost four years 13 minutes from time after Alex Berenguer had opened the scoring with an early header at San Mames.
Athletic were not to be denied victory, though, as Martinez rose to nod home four minutes from time and leave Diego Simeone's side shell-shocked.
The setback means Atletico are just two points clear of Real Madrid and Barcelona, with Ronald Koeman's side having a game in hand, while Sevilla are also within three points.
Berenguer opened the scoring only eight minutes in, planting a diving header beyond Jan Oblak from six yards out after Ander Capa's cross deflected off Felipe and looped into his path.
Unai Simon prevented Atleti from equalising moments later when he tipped Yannick Carrasco's header over the crossbar and Angel Correa blazed wildly wide after turning sharply in the penalty area.
Oblak kept out Asier Villalibre's shot at his near post and Capa drilled over with a venomous drive from the resulting corner before the Athletic full-back limped off in agony right at the end of an entertaining first half.
Atleti wanted a penalty for handball against Unai Nunez early in the second half, but their protests were met with shake of the head from referee Pablo Gonzalez Fuertes.
They looked to be running out of ideas until centre-back Savic came up with a collector's item, rising to head in Carrasco's corner.
Yet Atleti were made to pay for switching off in the closing stages, as an unmarked Martinez headed an Ibai Gomez corner powerfully beyond Oblak to win it.