Barcelona coach Hansi Flick praised Ferran Torres' ability to fill in for Robert Lewandowski after the forward netted a hat trick in Thursday's 5-0 Copa del Rey quarterfinal win over Valencia at Mestalla.

Fermín López and Lamine Yamal were also on target as Barça joined Real Madrid, Atlético Madrid and Real Sociedad in next Wednesday's semifinal draw.

Torres' treble took his tally for the season to 11 in all competitions, with Flick increasingly using the winger as a central striker to rest Lewandowski.

"It's important that everyone shows his best," Flick said in a news conference when asked about Barça not losing their nose for goals when Lewandowski is not on the pitch.

"Ferran, his performance was really good. As a No.9, it counts that you score goals and he did it really great today.

"I am happy because he scored three goals. It's amazing because he has not had an easy time this season. He came back after an injury and I'm happy for him, he was great today."

Torres was one of several players to come into the Barça side as Flick flexed his squad's depth, with López, Frenkie de Jong and Eric García also handed starts.

The changes didn't affect Barça's rhythm as they followed up beating Valencia 7-1 in LaLiga two weeks ago by racing into a 4-0 lead at Mestalla inside the first 30 minutes.

"Now it's important [to show that depth] because we have tough weeks and months until the end of the season," the German coach added.

"It's important everyone shows their best; how good they are. This helps us a lot. I am happy for the players coming in, scoring goals and also playing really good.

"I am happy like the match against Valencia at home [in LaLiga]. Today we started from the beginning very focused, created great chances and scored the goals."

It was a bittersweet hat-trick for Torres as it came against his former club, who are now out of the cup and fighting for their lives in LaLiga.

His third goal in the 30th minute was the cue for some Valencia supporters to head to the exits, while others stayed to protest the club's ownership.

"I wish them all the luck in the world, I hope they stay up because I'm still a fan," Torres told reporters after refusing to celebrate his goals.

"It's hard to see the club of your life suffer like this. They're going through a tough time. I hope the fans can forget all the non-sporting [issues] and get behind the players because they need it to stay up."

Carlos Corberán's side can now focus fully on LaLiga, where they are currently second bottom and four points from safety with 16 games to go.

"I apologise to the fans for giving them so little after having received so much [support] from them," Corberán said in a news conference.

"These fans don't deserve what happened today on the pitch. We were nowhere near the level required. We need to transform the pain we're feeling into a reaction in our next games."

Valencia host fellow strugglers Leganés in a huge league game at Mestalla on Sunday, while Barça travel to Sevilla later on the same day.