Ferran Torres's strike earned Barcelona a tense 1-0 win at Porto in the Champions League on Wednesday in coach Xavi Hernandez's 100th game in charge.
Barcelona have been eliminated in the opening stage in the past two editions but this hard-fought triumph on the road puts them in a strong position as Group H leaders to avoid a third consecutive strike-out.
Xavi said the trip to the north of Portugal would be the hardest group game and his team had to dig deep, surviving a penalty which was cancelled after a VAR review and an offside goal in the gripping final stages.
Barca midfielder Gavi was sent off in stoppage time for a second yellow card as the Catalans clung on desperately to secure victory.
"We had been talking about it, this match was worth half of the qualification," Torres told Movistar.
"It was a win that we had to get however we can, and we showed that we are good with the ball but we know how to suffer without it too.
"Football is about taking advantage of opposition mistakes – they put us under a lot of high pressure. The important thing in the end is the three points."
Xavi selected youngster Lamine Yamal on the right wing, making him the youngest starter in Champions League history at 16 years and 83 days.
The teenager showed some bright sparks in the first half but largely Porto were on top, cutting through a Barcelona midfield shorn of the injured Pedri and Frenkie de Jong.
Barcelona holding midfielder Oriol Romeu endured a tough night, with Porto harrying him at every possible moment, roared on by a packed Dragao stadium.
Brought in from Girona to replace Sergio Busquets, the job is proving a big one for the former Southampton and Chelsea man to handle.
He was fortunate to avoid a yellow card in the first half but his teammates Joao Cancelo, Ronald Araujo and Gavi went into the book – along with coach Xavi.
Mehdi Taremi appealed for a penalty early on after Jules Kounde took a handful of his shirt in the area but it would have been soft.
At the other end Joao Felix was involved in Barcelona's best attacking moments, to a cacophony of whistles from Porto fans who had not forgotten his Benfica past.
The Eagles beat Porto in the Portuguese Clasico last week, which Felix said would only motivate his old rivals even further.
However Porto midfielder Romario Baro lost concentration just before halftime and it proved costly for his side, who had more than held their own.
COSTLY ERROR
Romario's under-hit pass was easily stolen by Ilkay Gundogan, who threaded through Torres.
The Spanish forward, who replaced injured hitman Robert Lewandowski earlier in the first half, rolled home a low strike under Diogo Costa.
Jules Kounde and Joao Felix were booked early in the second half as Barcelona did everything they could to keep Porto at bay.
French centre-back Kounde made a sublime challenge to knock the ball away from Pepe from behind as he was poised to shoot, after running through on Barcelona's goal.
Taremi headed narrowly off target and Barcelona goalkeeper Marc-Andre ter Stegen made two good saves from Galeno as Porto turned the screw.
Referee Anthony Taylor awarded Porto a penalty with 10 minutes to go after Cancelo handled in the box, but after a VAR review, cancelled the decision for an earlier handball by Stephen Eustaquio.
Taremi then scored a brilliant overhead kick but he was offside as he met winger Pepe's cheeky scooped pass, with Barcelona holding on for the three points, despite Gavi's late dismissal.
