Barcelona were brought back down to earth with a thud as they lost 2-1 away to Deportivo La Coruna in LaLiga.
Four days after recording the biggest comeback in Champions league history with a 6-1 thrashing of Paris Saint-Germain, Barca were extremely poor as they fell to a costly defeat in the title race.
Joselu punished a first-half mistake from goalkeeper Marc-Andre ter Stegen to put hosts Depor ahead after his only other two league goals this season had come against Real Madrid.
Luis Suarez, who scored four when Barca won 8-0 at the Riazor last season, appeared to have got the visitors back on track after the break, but Depor captain Alex Bergantinos' header with 16 minutes remaining proved decisive.
Barca had won six league matches in a row coming into the fixture, but could now lose top spot in LaLiga on Sunday, with Real Madrid still just one point behind ahead of their home match against Real Betis.
Impressive Depor are now four matches unbeaten under new boss Pepe Mel, this victory opening up an eight-point gap on the relegation zone.
A quiet start to the match saw Joselu fire an effort straight at Ter Stegen in the only early opportunity of note.
Barcelona, who made five changes from the PSG triumph, did not threaten until Luis Suarez poorly sent a free header into the arms of German Lux after latching on to Lionel Messi's free-kick just before the half-hour mark.
Depor went close when a superb half-volley from Joselu was impressively kept out by Ter Stegen at full stretch, Fernando Navarro creating the chance.
But the hosts went one better from the resulting corner, Ter Stegen failed to gather Javier Mascerhano's attempted clearance, allowing Joselu to thump in from close range.
2 - Joselu is the second player to score against Real Madrid and Barcelona in La Liga this season, after Deyverson Silva. Stellar.-- OptaJose (@OptaJose) March 12, 2017
Barca wasted no time in response, Luis Suarez scoring just 59 seconds after the restart.
Denis Suarez's cross from the right was only half cleared by Alejandro Arribas and Luis Suarez sent a powerful finish past Lux at his near post to move on to 21 league goals this season.
Depor's Carles Gil had a penalty claim turned down after being challenged by Sergio Busquets, while Faycal Fajr's shot deflected just over as the hosts reacted well to the blow.
Luis Enrique sent on Ivan Rakitic and Andres Iniesta for Arda Turan and Andre Gomes in an attempt to seal victory, but the home side threatened again when Joselu's impressive volley tested Ter Stegen after Bruno Gama had led a fast break.
Denis Suarez had an effort saved for the visitors as they struggled to make an impact, while Depor forced Ter Stegen into action again through Arribas' bouncing header.
And from the resulting corner, Bergantinos got above Jordi Alba to head in substitute Emre Colak's delivery.
Paco Alcacer was thrown on in Barca's final change and the visitors pushed hard for an equaliser, Luis Suarez denied by Lux from close range and Messi also seeing a shot saved by the keeper.
Messi then wasted a good free-kick opportunity in a frantic stoppage-time period that saw Fajr race clean through on goal before seeing one shot saved by Ter Stegen and another hacked off the line by Alba.
Mel's men held on for a deserved victory, with Barca's hopes of retaining their title suffering a significant setback as they struggled without the injured Neymar.