Raul Garcia's superb 90th-minute strike consigned leaders Real Madrid to a 2-1 defeat at Osasuna in LaLiga on Saturday, in a blow to their title hopes.
Champions Barcelona trail Madrid by two points but can overtake them on Sunday if they beat Levante at Camp Nou.
Elsewhere, Atletico Madrid, fourth, got back to winning ways with an entertaining 4-2 victory over Espanyol.
Alvaro Arbeloa's Real Madrid put on a below-par performance in Pamplona and fell behind to an Ante Budimir penalty, before Vinicius Junior pulled the visitors level.
The second leg of Madrid's Champions League play-off against Jose Mourinho's Benfica on Wednesday seemed to play on their minds as they failed to kick on at El Sadar.
Vinicius, who scored a sensational goal to give Madrid a slender first-leg lead but was then the victim of alleged racial abuse, thought he had earned his team a point before Garcia blasted home the winner.
"It was not a good game from us... we have to play a lot better," Arbeloa told reporters.
"We had control at some points but we lacked speed in our play... on Wednesday we have another final, another game that is very important for us."
Croatian target man Budimir forced a good save from Real Madrid goalkeeper Thibaut Courtois and hit the post as he gave the visitors a host of problems.
The hosts, ninth, took the lead after Budimir burst into the area and went down under a challenge from Courtois.
Although he was initially booked for diving a VAR review showed the goalkeeper stood on his foot, and the veteran forward sent Courtois the wrong way from the spot.
"I knew it was a penalty and I wanted to stay cool (while it was being reviewed)," said Budimir.
Chasing an equaliser, Arbeloa threw on creative duo Trent Alexander-Arnold and Brahim Diaz and his team began to take control.
French superstar Kylian Mbappe put the ball in the back of Osasuna's net but had edged offside, although the equaliser was not long in coming.
Fede Valverde charged into the box and crossed for Vinicius to tuck home.
It was the Brazilian's fifth goal in his last four games across all competitions, having found his best form since Arbeloa replaced Xabi Alonso at the helm in January.
WASTED CHANCE
Mbappe, LaLiga's top goalscorer, should have put Madrid ahead with 10 minutes to go but Javi Galan brilliantly blocked his shot, and Los Blancos paid the price at the death.
Raul Garcia cut inside the sliding Raul Asencio brilliantly and fired across Courtois and in at the far post, with the goal initially ruled out but then proven to be onside with a VAR review.
It was the first time Osasuna have beaten Madrid in LaLiga for 15 years.
"It's an incredible feeling... I think (that fact) is crazy. The fans deserved a victory like this after so many years," Raul Garcia told DAZN.
Real Betis, fifth, were held 1-1 at home against Rayo Vallecano earlier Saturday, leaving them six points behind Atletico, after Diego Simeone's side put four past Espanyol.
Alexander Sorloth struck twice as Atletico earned their first victory in four league matches in a goal-riddled game at their Metropolitano stadium.
They trail leaders Real Madrid by 12 points, meaning their hopes of fighting for the title are virtually over, but strengthened their grip on a top four spot.
Atletico host Club Brugge on Tuesday in a crucial Champions League play-off second leg clash, following a 3-3 first leg draw in Belgium on Wednesday.
"We needed to get the points to close in on the teams above us," said Simeone.
"We were protagonists, we scored four goals, we had two or three more clear chances, we played a very complete game."
Espanyol took the lead in an open game through Jofre after six minutes but Sorloth pulled Atletico level midway through the first half.
The coach's son Giuliano Simeone netted soon after the interval to give the Rojiblancos the lead.
Nigeria international Ademola Lookman grabbed the third from close range before Sorloth nodded in a fourth as the game flowed from end to end.
Edu Exposito thrashed home another for Espanyol from long range but it was scant consolation for a side who started the season brilliantly but are now without a win in eight, although they remain sixth.