Inigo Martinez's late header struck a blow to Atletico Madrid's LaLiga title hopes as Athletic Bilbao secured a 2-1 victory on Sunday.
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 minute 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 shellshocked.
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 referee 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.
What does it mean? Slack Atleti slip up
Simeone warned his side they could not afford to relax with so much at stake and they will have to regroup following this disappointment.
It is now four away games without a victory for Atleti, which does not bode well with a trip to Barcelona to come in a fortnight.
The return of Luis Suarez was a plus, but this could yet prove to be costly defeat with just five games remaining. The win for Athletic ended a run of five consecutive LaLiga draws.
Martinez makes big impact at both ends
Centre-back Martinez will get the headlines for his winning goal, but he played a big role at the other end as the Atleti attack misfired.
The leaders had scored seven goals in their previous two games without conceding, but Martinez and his fellow Atleti defenders kept their forward line quite for much of the contest.
Correa out of sorts
Correa had been involved five goals in the previous three games for the capital club, but the Argentine forward endured an off night on this occasion.
He failed to hit the target with three attempts before being replaced by Lucas Torreira after 65 minutes.
Atleti are on their travels again to take on relegation-threatened Elche next Saturday, while mid-table Athletic are on the road to face fourth-placed Sevilla a week on Monday.