Roberto Soldado scored in the 55th minute to give Osasuna a slender advantage after the first leg of their Uefa Cup semi-final with Sevilla.
Scorer Roberto Soldado (left) is challenged by Javi Navarro
Soldado headed in from David Lopez's free-kick to separate the teams.
Osasuna will be rueful that the match was stopped for seven minutes in the second half when the referee suffered an injury, just as they were on top.
The game restarted with the fourth official in charge but neither side could get into their rhythm thereafter.
Osasuna had the better of the chances - with Lopez, Soldado and striker Savo Milosevic all going close - but they could not make the most of a purple patch just before the game was interrupted.
Dutch referee Eric Braamhaar tore a muscle in his calf and was eventually replaced by fourth official Pieter Vink in charge.
Substitute Alexander Kerzhakov was inches away from equalising for Sevilla late on with a superb flicked volley - but his side were otherwise below par on the night.
Osasuna coach Jose Angel Ziganda:
"You don't often see Sevilla as uncomfortable as it was tonight, so we have to be happy because it's a good result.
"The match is difficult to predict and even more so because our opponents are as good a team as Sevilla."
Sevilla coach Juande Ramos:
"We struggled to pick up the pace of the game. We tried to create chances at the end but our offensive input was much less that you would usually get from Sevilla.
"They are a goal ahead but we now have home advantage. We're at half-time and we hope we can turn the situation around."
Osasuna: Ricardo, Izquierdo, Cuellar, Raul Garcia, Cruchaga, Milosevic (Valdo 87), Punal, Juanfran, David Lopez, Soldado (Webo 81), Corrales.
Subs Not Used: Elia, Josetxo, Javier Flano, Munoz, Erice.
Booked: Soldado, Cuellar. Goals: Soldado 55.
Sevilla: Cobeno, Javi Navarro, Daniel, Adriano Correia (Puerta 90), Poulsen, Luis Fabiano (Kerzhakov 61), Kanoute, Escude, Jesus Navas, Marti (Hinkel 76), Dragutinovic.
Subs Not Used: Palop, Chevanton, Aitor Ocio, Maresca.
Booked: Poulsen, Kanoute, Luis Fabiano.
Ref: Erik Braamhaar (Netherlands).