The Catalans survived a draining Champions League semi-final second leg away to Chelsea last Wednesday, scoring a late equaliser to book a place in the May 27 final against Manchester United.
But on Sunday, they were on the receiving end of a stoppage-time goal that denied them the Primera Liga title at home to Villarreal. Barca let slip a 3-1 lead to draw 3-3 following the red card for defender Eric Abidal, 12 minutes from time.
Joseba Llorente's strike silenced a packed Nou Camp who had already started their celebrations. Barca only need one point from their remaining three matches, but the look of disbelief on his players' faces will have worried coach Pep Guardiola.
"A few days ago we did the same to Chelsea, now it is us who have suffered. But we are closer still to the title," Guardiola said.
"We have to pick ourselves up, recover our smiles, and prepare the best we can for the cup final."
Guardiola said his players were feeling the accumulated strain of so many important matches in such a short space of time, and was particularly concerned by the injury to midfielder Andres Iniesta who damaged a thigh muscle in Sunday's game.
Iniesta joins Thierry Henry and Rafael Marquez on a growing injury list, and he will also be deprived of the suspended Abidal.
The Frenchman's second red card in as many games ruled him out of his second final but Barca said on Monday they would appeal to free him for the King's Cup final. Barca have already said they will appeal the red card Abidal picked up in the Champions League semi-final which has ruled him out of the May 27 final.
WAVE OF EMOTION
Barca won a record 24th King's Cup on their last final appearance against Real Mallorca in 1998, which was also held in Valencia's Mestalla, the venue for Wednesday's encounter.
Up against them are a side who are being swept to the east coast on a wave of emotion by their fans -- 20,000 of whom packed San Mames on Sunday just to watch their last training session before they travelled.
Athletic rested most of their first-team regulars for the 1-0 victory over Real Betis on Saturday, that left them safe in mid-table.
Athletic won the last of their 23 cups in 1984, beating Barcelona 1-0 in a final best remembered for a brutal exchange of kicks and punches between the players, including Diego Maradona, at the end.
Bilbao only field players from the Basque region, and coach Joaquin Caparros hoped a victory over Barca would help prove that their model can still cut it at the highest level. It has been under scrutiny after recent battles to avoid relegation.
"If we could win a final against Barcelona, who are the best team in the world at present, it would make Bilbao's unique model the centre of world attention. The chance to do this gives the footballers an extra motivation," he told sports daily Marca.
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