Barcelona's La Liga title defence took a blow when they stumbled to a 1-0 defeat away to Real Valladolid on Saturday.
Atletico Madrid took advantage by winning at Celta Vigo to move ahead of Barcelona and level on points with Real Madrid at the top of the table.
Once the league pace-setters, Barcelona have won just two of their past seven away league games and trail their title rivals by one point before Real Madrid host Levante on Sunday.
While Lionel Messi looked sharp, his teammates were plagued by uncharacteristic passing errors and proved brittle at the back. Sergio Busquets erred on three passes before the 25-minute mark.
Valladolid outplayed the defending champions early and midfielder Fausto Rossi netted the winner in the 17th minute. The relegation-threatened side then did well to protect three points against the star-studded visitors.
Messi and Cesc Fabregas forced Diego Marino to make three more saves before half-time. After the break, Messi used a burst of speed to break free in the 48th minute, only to be denied again by Marino.
Neymar had a quiet match and sailed his only scoring chance over the bar before he was substituted.
Atletico Madrid won 2-0 at Celta Vigo courtesy of David Villa's two goals in a three-minute span.
Atletico were without leading scorer Diego Costa, playmaker Arda Turan and centerback Diego Godin, all to suspensions.
That meant Villa, who was recently dropped from Spain's squad for their friendly with Italy, got the nod up front. And after a dull first half, the former Barcelona striker notched his 12th and 13th league goals of the campaign.
Villa first pounced on a poor pass by Celta's Jonathan Castro, dodged Andreu Fontas' last-ditch tackle, and drilled the ball beyond goalkeeper Yoel Rodriguez in the 62nd minute. He slotted in his second goal to seal the victory two minutes later.
Villarreal lost 2-0 at Granada to falter in the race for fourth place.
Fran Rico opened the scoring with a 23rd-minute penalty and Youssef El Arabi doubled the lead 10 minutes later.
Last-place Real Betis gave a boost to their slim hopes of staying up by beating Getafe 2-0 in Seville.
Betis won thanks to first-half goals from forwards Leo Baptistao and Ruben Castro to keep the hapless Getafe winless in 12 consecutive rounds and only one point from relegation.
Information from The Associated Press was used in this report.