MADRID // Barcelona missed a chance to stretch their advantage over chasing Real Madrid and virtually wrap up a third straight La Liga title when they surrendered the lead and lost 2-1 at Real Sociedad.
With Madrid facing the task of overturning a 2-0 deficit in Tuesday's Champions League semi-final second leg at Camp Nou, Real coach Jose Mourinho rested key players.
His team, without top scorer Cristiano Ronaldo among others, was upset 3-2 at home by relegation-threatened Real Zaragoza.
Leaders Barca looked in good shape to open an 11-point lead over their arch rivals with four games remaining when midfielder Thiago Alcantara opened the scoring in the 29th minute at mid-table Sociedad's Anoeta stadium in San Sebastian.
But Diego Ifran pounced on a loose ball to equalise with less than 20 minutes left and Xabi Prieto netted from the spot in the 82nd after substitute Javier Mascherano conceded a penalty.
The result, only the second defeat of the season for Pep Guardiola's side, meant Barca just missed equalling Sociedad's record run of 32 La Liga games unbeaten in the 1979/80 season and leaves them on 88 points from 34 matches.
Real, who beat Barca 1-0 to win a dramatic King's Cup final last week, have 80 points in second.
"We are a team that can win or lose," Guardiola said at a news conference.
"We were coming into the game after a very complicated situation," he added, referring to the controversy around Wednesday's ill-tempered Champions League semi-final first leg, which resulted in UEFA opening a probe into both teams.
"To be champions we will have to get the four points that we still need."
If Real managed to draw level on points, Barca would still win the title due to a superior head-to-head record.
They routed Real 5-0 at the Nou Camp in November and drew 1-1 at the Bernabeu earlier this month.
Real's city rivals Atletico Madrid stayed on course to qualify for the Europa League they won last season when Sergio Aguero stabbed home in the 80th minute to snatch a 1-0 win at 10-man Deportivo Coruna in the late kick-off.
Atletico climbed to fifth on 52 points, one ahead of Athletic Bilbao, who play at eighth-placed Espanyol on Monday.
Depor, who had Alberto Lopo sent off for a second yellow card in the 59th minute, are 14th, three points above the relegation places.
An uncharacteristic error by Real goalkeeper and captain Iker Casillas, who presented the King's Cup to the home fans braving the rain in the Spanish capital before kick-off, gifted Zaragoza their opening goal four minutes before halftime.
The Spanish World Cup winner galloped out to clear but failed to connect. After a miscued effort by Ikechukwu Uche, the ball fell to Angel Lafita who rolled it into the empty net.
Gabi doubled the visitors' lead from the penalty spot 10 minutes into the second half after Ricardo Carvalho brought Lafita down in the area before Sergio Ramos pulled a goal back when he headed home from a corner.
Angel Di Maria, Mesut Ozil and Marcelo started on the bench and Real looked infinitely more threatening when the trio were introduced early in the second half.
They dominated possession and pressed for an equaliser but Lafita grabbed his second of the night when he finished off a fine counter-attack with just over 10 minutes remaining.
After Karim Benzema pulled another goal back with a low, angled strike in the 85th minute, Ricardo Carvalho earned a second yellow card and was sent off.
Zaragoza held on for a potentially priceless three points that lifted them to 13th in a congested lower half of the table ahead of Sunday's matches. Sociedad are up to to 11th.