Lionel Messi scored twice as Barcelona ruined Marcelino's first match in charge of Athletic Bilbao with a hard-fought 3-2 triumph at San Mames.
Inaki Williams made an instant impression on Athletic's new boss - appointed following the dismissal of Gaizka Garitano - by scoring inside three minutes for the home team on Thursday (AEDT).
Capitalising on a ridiculously high defensive line, the forward ran clear from inside his own half and, after cutting back inside a half-hearted challenge from Clement Lenglet, fired low past Marc-Andre ter Stegen to break the deadlock.
However, with Messi leading the way, Barca recovered from the setback to turn the game around by the interval, albeit with some help from the hosts.
Unai Simon was tempted out to deal with Messi's seemingly over-hit cross for the equaliser, the alert Frenkie de Jong stretching out to volley the ball back into the middle for Pedri to nod into an unguarded net.
The Athletic goalkeeper was caught again when his side conceded seven minutes before the break, Pedri the provider as his clever back-heeled assist set up the supporting Messi to pass the ball in, Simon - who had initially rushed out expecting the teenager to shoot - stranded out of position.
Athletic briefly threatened a recovery of their own early in the second half, yet it was always their opponents who carried the far greater threat in attack.
Having seen a goal ruled out for offside and a long-range shot denied by a post, Messi finally scored again when sweeping Antoine Griezmann's square pass home via the underside of the crossbar.
There was late drama when a turnover in possession allowed Alex Berenguer to tee up Iker Muniain for a first-time finish that cut the gap in the 90th minute, but Barca held firm to claim all three points.