RSC Anderlecht will head into next week's second leg against Athletic Club favourites to progress to the UEFA Europa League round of 16 after coming away from Spain with a 1-1 draw.
Ariël Jacobs' visitors, the Belgian league leaders, assumed the ascendancy courtesy of Lucas Biglia but were on the back foot for large swathes of the game. They could have no complaints about Mikel San José's opportunist second-half leveller for the hosts.
Defensive laxness appeared Athletic's likely undoing from the off, Jonathan Legear and Mbark Boussoufa both catching the hosts' back line unawares inside the opening half-hour only to be foiled by Gorka Iraizoz. The Spanish goalkeeper's next contribution was equally telling, Romelu Lukaku – himself guilty of snatching at an earlier chance – seeing his tame effort inexplicably spilled at the feet of Biglia, who was alert enough to convert.
Silvio Proto's palms had barely been warmed by Athletic but he remained vigilant enough to smother Oscar de Marcos's effort in first-half added time after neat, and long overdue, build-up play from Fernando Llorente and Francisco Yeste. Such penetration threatened to be an anomaly, but an uneventful opening to the second period proved the calm before a brief storm, San José poking in Markel Susaeta's cross-shot to ignite the home side, the crowd and the tie.
Llorente, top scorer in the group stage with six goals, should have added to both that tally and the growing sense of optimism in Bilbao moments later but recklessly headed wide. Then came the aftermath, both sides, on the surface at least, seemingly content to go to Belgium on level terms, Llorente wasting the most notable chance of a potentially defining goal.