Valencia's Europa League campaign is hanging in the balance after they lost 1-0 to Athletic Bilbao in torrential rain at San Mames.
Raul Garcia's first-half header put the hosts in front and Gary Neville's side never recovered, showing little of the flowing attacking play that saw them crush Rapid Vienna in the previous round.
The downpour did not prevent Bilbao from peppering the Valencia goal with shots throughout the game, and the tie might have been put beyond doubt had it not been for the good form of visiting goalkeeper Mathew Ryan.
With neither side in contention for a place in next season's Champions League via La Liga, the Europa League holds the best chance of a place in next season's top competition for both these sides.
Bilbao could still clinch a top six finish domestically to earn another Europa League spot for next season, but Neville's men will need to be at their best at the Mestalla if they are to keep their European campaign alive.
A day of rain in Bilbao had put the game in doubt but, after treatment by ground staff before kick-off, referee Bjorn Kuipers deemed the pitch playable and Valencia went took to the field with nine changes from the side that lost 3-1 to Atletico Madrid in their previous game.
Ryan was called into action early on to punch clear a header from the Europa League's top scorer, Aritz Aduriz, and the Australia international was alert again moments later to save a shot from Markel Susaeta.
Bilbao's dominance paid off when Benat whipped in a free-kick from a deep position and Garcia stretched to win a header that flew past Ryan into the net.
Moments later Aduriz beat Shkodran Mustafi in the air and had the goal at his mercy, but the in-form striker spurned the chance and put his shot wide.
Valencia struggled to adapt to the heavy conditions, and mustered little other than a Dani Parejo shot that failed to test Iago Herrerin in the Athletic goal, but at the other end Ryan did well to hang onto a fizzing effort from Oscar de Marcos.
Athletic continued to apply the pressure after half-time, and defender Aymeric Laporte attempted an audacious lob from all of 45 yards that had Ryan back-pedalling before he tipped it over the crossbar.
The home side had claims for a penalty turned away when Mikel Balenziaga's shot appeared to strike Javi Fuego on the arm, and Aduriz missed another chance to add to his Europa League tally of seven this season, firing a header straight at Ryan when unmarked in front of goal.
Ryan made the best of his many saves of the night from a Bernat free-kick, which he dived to his left and palmed clear of danger, but Valencia will need more than just an in-form goalkeeper to turn the tie in their favour in the second leg.