Barcelona were outclassed by Real Zaragoza as they went down 1-0 at La Romareda on Saturday in a defeat which will give their title rivals renewed hope of depriving the Catalans of a third successive league title.
Diego Milito took credit for the goal to take his tally for the season to 19 after claiming he got a touch to an acute-angled shot from fellow Argentine Andres D'Alessandro 13 minutes into the second half.
The scoreline did little to reflect the domination of a Zaragoza side that had Barcelona on the ropes for most of the game.
Barca coach Frank Rijkaard's decision to opt for a three-man defence played into Zaragoza's hands from the start and his side were fortunate not to concede several more goals.
The Dutchman rectified his decision in the second half, but by then Zaragoza had already gained an unstoppable momentum that meant their opponents could do little to stem the tide.
Barca have now won just one of their last nine away matches and although they remain top of the table on 56 points, they are only two clear of Sevilla and five ahead of arch-rivals Real Madrid who both have a game in hand.
Zaragoza moved up to fourth on 50 points, after Valencia lost 1-0 at relegation-threatened Athletic Bilbao.
Valencia coach Quique Sanchez Flores clearly had one eye on next week's Champions League quarter-final return against Chelsea as he decided to leave a host of first-choice players on the bench.
Bilbao, struggling to avoid dropping out of the top flight for the first time in their history, took full advantage and grabbed a vital win thanks to a stylish finish from Igor Gabilondo.
The midfielder clipped a clever back-heeled shot past reserve keeper Ludovic Butelle after some great work by winger Joseba Etxeberria on the right just before the half hour to give the Basques their first win in six and lift them out of the drop zone.
RISKY STRATEGY
Rijkaard's decision to opt for a three-man defence looked like a risky strategy, especially when his side sat back and handed the initiative to Zaragoza in the opening minutes.
D'Alessandro gave Barca an early warning of the threat to come when he smashed a volley over the bar in the first attack.
Uruguay international Carlos Diogo was only denied by a sharp block from Victor Valdes and former Barca striker Sergio Garcia gave his old side another scare when he grazed the post on the half hour.
Barca's only real shot of the first period came when Lionel Messi fired straight at Cesar after a rare foray into the Zaragoza area.
The champions somehow managed to survive until the break when Rijkaard decided to change plan and throw on Italian full back Gianluca Zambrotta to form part of a four-man defence.
But it was to no avail as D'Alessandro cashed in on some poor defending from Oleguer and Lilian Thuram after a throw-in to fire an angled shot between the two players from the byeline.
Milito, who was lurking in front of Barca keeper Valdes, claimed the goal.
Barca momentarily sparked into life, with Deco being denied by a sharp block from Cesar and Eidur Gudjohnsen firing wide after snapping up a rebound from a Messi shot, but it was Zaragoza who remained in charge until the final whistle.
