Barcelona face a real uphill battle in the second leg of the Spanish Super Cup after suffering a shock 4-0 defeat at the hands of Basque rivals Athletic Club at the San Mames Stadium on Friday night.
Neymar absent again due to mumps
Pedro handed Barca starting berth
San Jose stunner breaks deadlock
Aduriz nets 15-minute hat-trick
A Mikel San Jose wonder strike was all that separated the sides at the half-time break as Barcelona struggled to impose themselves on proceedings.
And it went from bad to worse after the interval as Aritz Aduriz netted a superb headed goal before notching up his second of the evening with a deflected strike.
Aduriz went on to convert from the penalty spot as he completed a remarkable 15-minute hat-trick against the defending La Liga champions.
The match started at a cautious tempo, with a much-changed Barcelona content to keep possession early on without venturing too far forward.
It was the hosts who had the first real chance of note, though, as striker Aritz Aduriz forced goalkeeper Andre Ter Stegen into a fingertip save before French defender Aymeric Laporte headed over from a corner.
The two quick-fire chances gave the hosts a real belief that a positive result was possible, while an unfamiliar Barca line-up, containing Manchester United transfer target Pedro, struggled to play with any real fluency.
The opening goal arrived on 13 minutes like a bolt out of the blue. Ter Stegen raced off his line to head clear a dangerous ball over the top, but the ball fell only as far as San Jose. The midfield lynchpin needed no second invitation, hitting a first-time volley into the back of the net from roughly 45 yards out.
Athletic Club's early goal left Barca rattled as they struggled to break down a well organised defence after falling behind early on.
A rare chance arrived for Barcelona in the 27th minute when Pedro made a surging run on the counter-attack before attempting a cross-cum-shot toward the back post, where San Jose was waiting to head the ball clear.
Aduriz was causing the Barca defence all sorts of problems as the half wore on, and went close to doubling his side's advantage on two separate occasions.
However, it was the visitors who very nearly netted on the stroke of half-time as Lionel Messi saw his dipping free-kick brilliantly kept out by Gorka Iraizoz in the Bilbao goal.
Barca picked up where they left off after the half-time interval, with Pedro hitting the bar on 50 minutes and Sergi Roberto heading into the grateful arms of Iraizoz on the follow-up.
Iraizoz was again called into action one minute later, doing well to quickly get down and keep out Luis Suarez's curling effort with a fingertip save.
However, Bilbao hit back with a chance of their own just moments later and doubled their advantage, much to the delight of the boisterous home support. Aduriz was the man to add his name to the scoresheet, leaping above his marker to head home from Sabin Merino's inviting left-wing cross.
Barca were still reeling from the second goal when the hosts struck again in the 62nd minute, Aduriz doubling his tally with a deflected strike into the bottom corner from the centre of the area.
The visitors' defence had looked suspect all evening and they were again found wanting in the 67th minute when Dani Alves was adjudged to have fouled defender Xabier Etxeita at the near post. Aduriz was tasked with taking the resulting spot-kick and made no mistake, slotting the ball into the bottom-left corner.
Messi had a glorious opportunity to pull one back for the visitors just two minutes later, but the striker blasted high and wide from the edge of the box
Barca kept pushing numbers forward in the closing stages of the match, but an obstinate Bilbao defence held firm, with Suarez cutting a frustrated figure on the front line.
The Uruguayan fired high and wide on 84 minutes after being teed up by substitute Andres Iniesta, but that was the last real chance of note as Athletic Club held on for a famous victory on home soil.