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."
Ernesto Valverde brought defender Inigo Martinez into the Athletic starting line-up after injury, for his first appearance since November.
Abde, who scored the winner in Osasuna's 3-2 win over Sevilla in La Liga last weekend, fizzed an early deflected strike wide of the far post.
At the other end, Inaki Williams finished well but his goal was disallowed for a clear offside.
The first half offered few clear chances, in a cagey encounter, but Abde broke through early in the second half.
Moi Gomez found the Morocco international, who brilliantly beat defender Daniel Vivian and drilled a low effort across goal and into the far corner.
It proved the difference between the sides, with Osasuna holding off Athletic in the nervy final stages.
Sergio Herrera made a solid save late on to keep out an effort from Athletic striker Gorka Guruzeta at his near post.
"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.