Jose Angel Ziganda's side, who have never previously reached this stage of the competition, enjoyed the perfect start when Carlos Cuellar headed them into the lead in the first minute.

Leverkusen were then caught out by two goals in as many second-half minutes through David Lopez Moreno and Pierre Webo, and barring a calamity in the second leg will advance to a semi-final against Primera Liga rivals Sevilla or Tottenham.

It took less than a minute for the visitors to find the breakthrough, Cuellar powerfully heading home Lopez Moreno's outswinging corner.

In the 72nd minute Osasuna were 2-0 up, Lopez Moreno slotting home a left-wing cross from former Aston Villa striker Savo Milosevic.

Before Leverkusen had chance to reply, Webo drilled a left-footed shot high into the centre of Adler's goal to cap a great night for the visitors.

Osasuna, who have an impressive record against Bundesliga opposition and remain unbeaten on German soil - after defeating Stuttgart in 1991's UEFA Cup and recording three other draws - could even have had a couple more goals.

Cuellar headed wide and Milosevic and Webo fired over, but Osasuna will be more than satisfied with their evening's work.