Athletic Bilbao cannot "logically" consider the Supercopa de Espana decided, according to coach Ernesto Valverde, who warned Barcelona are capable of anything at Camp Nou.
Bilbao crushed Barca 4-0 at home in the first leg of the Supercopa on Friday, with Mikel San Jose scoring a stunning 45-yard goal before Aritz Aduriz produced a second-half hat-trick.
It was just Bilbao's second win over the Catalan club since 2006.
"It's been a magical night here in San Mames," Valverde said at his post-match media conference.
"We're going to see if we're capable of defending it on Monday, there's still a lot ahead of us."
With the second leg of the Supercopa to be held in Barcelona, Valverde argued his team still have plenty of work to do.
"No, come on... we haven't won, neither have they, everyone's waiting for what happens at the Camp Nou," the 51-year-old said.
"So we have to go there to play. Whatever happens, it will be difficult to play the game there, we have an advantage and against another team you might think it would be enough but against Barca, logically, we know that's not true."
Bilbao have not defeated Barcelona on their home turf since 2001 - with the last five meetings at the Camp Nou ending in a victory for the hosts.
But having watched his side rip Luis Enrique's teetering defence apart at San Mames, Valverde argued his team will be confident when they step on the pitch for Monday's second leg.
If Bilbao can convert their four-goal aggregate lead into an overall victory, they will claim their first piece of silverware since the 1984 Supercopa, which came after the Basque club clinched the La Liga-Copa del Rey double in 1983-84.
"Nothing is finished because Barca are capable of anything, and scoring lots of goals, so we have to play our options and then whatever happens, try to take advantage of it," he said.
"But, yes, this is a result that strengthens us, but we have many hard moments ahead of us."