Athletic Bilbao will have the upper hand back at San Mames after holding Napoli to a 1-1 draw.
The Primera Division and Serie A giants were pitted against each other in a battle to reach the group stages and Athletic took the initiative with Iker Muniain scoring four minutes before the break in Naples.
Napoli hit back after the interval as former Real Madrid striker Gonzalo Higuain fired a leveller on 68 minutes but the Basque outfit will fancy their chances of winning the second leg and progressing to the next stage.
There were five goals in the first half of action in the Danish capital as Bayer Leverkusen claimed a 3-2 win at FC Copenhagen.
Stefan Kiessling (five) and Zanka (nine) exchanged goals before Daniel Amartey nudged FCK ahead in the 13th minute.
Bayer equalised through Karim Bellarabi on 31 minutes and, three minutes before the break, Son Heung-min grabbed what proved to be the winner for the Bundesliga giants.
Red Bull Salzburg will head to Sweden next week with their Champions League destiny in their own hands after Franz Schiemer and Jonathan Soriano scored in a 2-1 win over Malmo.
The ambitious Austrians, who conceded a 90th-minute away goal thanks to Emil Forsberg, were a surprise package in the Europa League last season and look set to cause more shock waves this year.
Steaua Bucharest have the advantage in their pairing with Ludogorets of Bulgaria after Alexsandru Chipciu fired the Romanians to a 1-0 victory at the Stadionul Steaua.