Real Sociedad damaged Barcelona's title defence ambitions as Goncalo Guedes gave them a surprise 2-1 La Liga victory on Sunday, consigning Hansi Flick's side to a first defeat in 12 matches.
Alexander Sorloth's towering header earned Atletico Madrid a narrow 1-0 win over Alaves on Sunday in La Liga, helping them pull level on points with third-placed Villarreal.
Barcelona, who hit the woodwork four times and had two goals disallowed, now only lead rivals Real Madrid by a single point at the top of the table after Alvaro Arbeloa's side beat Levante on Saturday.
Sociedad's Mikel Oyarzabal put the hosts ahead and immediately after Marcus Rashford equalised for Barcelona, Guedes struck the winner.
La Real, now unbeaten in four games under new American coach Pellegrino Matarazzo, had Carlos Soler sent off late on but managed to hold on to triumph and climb up to eighth.
"I said to the team we played good, we were unlucky. Some days are like this and you don't know why you lose but this is one day and now we have to focus on the next game," Flick told reporters.
La Real goalkeeper Alex Remiro made several crucial saves to keep Barca at bay.
"Three great points and a great game," said Remiro.
"We've started this year in great form... this is the energy we have, how we've changed (recently)."
In a frantic start to a compelling game at the rainy Reale Arena, both teams had goals ruled out.
La Real had the ball in the net inside 30 seconds when Oyarzabal headed home Guedes's cross, but had strayed offside.
At the other end, Barcelona's Fermin Lopez drilled home from range but Dani Olmo had committed a foul in the build-up and it was ruled out.
With teenage winger Lamine Yamal at his tricky best, Barca got in often down the right flank.
Yamal had a goal ruled out himself for an extremely tight offside call, before Sociedad took the lead against the run of play.
Oyarzabal lashed home Guedes's cross with a blistering volley inside Joan Garcia's near post.
Remiro heroics
Barcelona turned up the pressure in the second half and Real Sociedad were left depending on Remiro and the frame of the goal to keep the Catalans at bay.
Olmo crashed a shot against the post from Lopez's cross and then Remiro denied the Barca midfielder with a fine near-post save.
Remiro then tipped substitute Robert Lewandowski's header against the crossbar with the save of the night.
Another Barca substitute, Rashford, eventually pulled the champions level, heading Yamal's cross home after 70 minutes.
However, the hosts immediately regained their advantage after Barca goalkeeper Garcia spilled Carlos Soler's shot. The midfielder was able to gather the rebound and cross for Guedes, who fired La Real ahead again.
They should have got a third when Garcia, out of his goal, had no chance of saving Oyarzabal's effort but Pau Cubarsi headed the ball off the line.
Barcelona came within inches of an equaliser when Joao Cancelo, on his second debut for the club, crossed for Jules Kounde, who headed against the crossbar.
Soler was dismissed for an ugly foul on Pedri before nine minutes of stoppage time were added on.
Rashford hit the post directly from a corner kick as Barca pushed until the end but could not find a way through.
"I think we deserved to win today, we had a lot of chances, but in the end you have to put them away," Barca midfielder Frenkie de Jong told DAZN, before complaining about referee Jesus Gil Manzano.
The midfielder said he thought the referee was arrogant, and Flick agreed.
"I don't want to waste energy on him," said Flick, referring to Gil Manzano. "Frenkie is right."
La Real coach Matarazzo hailed his team's "heart", but admitted that his side had some fortune and was grateful for Remiro's saves.
"A quality goalkeeper is not luck, so it's a mix," he said.
After dropping points in a draw at Sociedad in their last league outing, Diego Simeone's Atletico got back on track with Sorloth's goal, moving seven points clear of Espanyol in fifth.
After a tight first half, Atletico took the lead early in the second, with Sorloth leaping high to nod home Pablo Barrios's cross and break the deadlock.
It was the Norwegian target man's third goal in his last three games across all competitions.
"(Sorloth) is in his most consistent spell since joining the club," said Atletico coach Simeone.
Source: AFP