Real Madrid's draw with Real Betis on Saturday gave Atletico the chance to pull further clear but instead they ended the weekend with their lead reduced
Atletico Madrid's title hopes suffered a setback on Sunday as a shock 2-1 defeat at Athletic Bilbao handed Barcelona the upper hand in a thrilling race at the top of La Liga.
Real Madrid's draw with Real Betis on Saturday gave Atletico the chance to pull further clear but instead they ended the weekend with their lead reduced, a loss at San Mames the latest twist in Europe's most unpredictable domestic chase.
Barca had earlier beaten Villarreal by the same scoreline, with Antoine Griezmann's quick-fire double becoming all the more valuable after another Atletico slip.
Sevilla have largely been overlooked as possible champions in Spain but they are right in the mix as well now after a 2-1 win over Granada puts them just three points off the top and one behind Real Madrid.
Yet Barcelona were the biggest winners, with Ronald Koeman's team able to go top of the table if they win their game in hand at home to Granada on Thursday, even if Atletico might have a way back given they are still to play at Camp Nou on May 8.
Real Madrid host Sevilla a day later.
"There are four of us in that race and now emotionally, those who have the mental strength will be closer to winning," said Atletico coach Diego Simeone.
"We still know that we depend on ourselves."
Atletico have first place, for now at least, but they might be the least convincing of all the contenders.
Losing to Athletic Bilbao means they have won only six of their last 14 league games, with the momentum gained from consecutive wins over Eibar and Huesca proving short-lived.
The way some of Simeone's players dropped to their knees at the final whistle suggested this result, psychologically, could be difficult to recover from.
Alex Berengeur had already nodded in Ander Capa's cross in the eighth minute and Athletic should have doubled their advantage in the second half but Oihan Sancet fired over.
Luis Suarez, Joao Felix and Thomas Lemar all came on and Atletico grabbed a lifeline when Stefan Savic's flicked header found its way inside the back post with 13 minutes left.
Simeone pumped his fists, urging his players to seek a winner, but instead there was a sucker-punch from Athletic, Inigo Martinez darting free in the box and powering a superb header past Jan Oblak with four minutes remaining.
- Griezmann fires Barca -
Barcelona are surely now favourites to add a La Liga title to the Copa del Rey they won earlier this month.
Villarreal struck first through Samuel Chukwueze but Griezmann equalised with a sumptuous lob before capitalising on a misplaced backpass to make it two.
Helped by a red card for Villarreal's Manu Trigueros shortly after the hour, Barcelona held on for a crucial win.
"The players are hungry to win another trophy," said Koeman.
"The league is closer, I don't know how much closer, but it is. We are on a good run and if we win the six games left, we will be champions."
Griezmann started on the bench for the defeat by Real Madrid but has been recalled for the last three games in a row, repaying the faith with a goal now in each.
It remains to be seen whether Barcelona see the 30-year-old as a chance to raise funds in the summer, and also whether the offers are there, but Griezmann now has 23 goals this season, and 15 since the turn of the year.
Sevilla, meanwhile, pushed their case for the title and guaranteed their place in the top four but their win over Granada will be remembered for the bizarre way it ended.
Both teams were made to come back out and play another minute after referee Ricardo de Burgos admitted he blew too early for full-time.
Sevilla's players had already left and some of them had started to take their kit off, with midfielder Fernando pictured without his shirt on in the stands, shaking his head in disbelief.
Marcos Acuna had to put his boots, socks and shinpads back on.
"The referee stopped it because he thought time was up and then he told us he'd made a mistake," said Sevilla's Lucas Ocampos.
"It was finished. I've never seen anything like it. We had basically taken our kit off."
Sevilla coach Julen Lopetegui said: "The really important thing is Sevilla has achieved something historic. The effort of these players has been extraordinary."