Ez Abde's fine solo goal earned Osasuna a slim 1-0 Copa del Rey semi-final first leg win over Athletic Bilbao on Wednesday.
The on-loan Barcelona winger burst into the box early in the second half and drilled home superbly to separate the teams in an even clash at El Sadar.
Osasuna, who last reached the semi-finals 18 years ago, eventually losing in the final against Real Betis, will take a narrow lead to the San Mames for the second leg on April 4.
Athletic have won the cup 23 times, more than any other side apart from Barcelona, and were playing in the semi-finals for the fourth consecutive season.
However, the last time they won the competition was back in 1984, and the Basque side have lost six times in finals since then.
"It was a very even game, and that start to the second half and moment of genius from Abde gives us a little advantage," said Osasuna coach Jagoba Arrasate.
"In general the two teams neutralised each other well, and there weren't many opportunities."
"The game was scrappy, I don't know if it was because both teams had a lot of respect for each other, but there was no one team who kept the ball," said Athletic coach Valverde.
"The game was broken up with Abde's goal, we threw ourselves into it (after that), we went forward more and more, but it wasn't to be."
Real Madrid host rivals Barcelona in the other semi-final first leg on Thursday.
