Flick said it was one of those comebacks that nobody forgets easily, adding it was not easy to win after Borja Iglesias netted a hat-trick to give Celta a 3-1 lead.
Barcelona will be aiming to open up a seven-point lead at the top of LaLiga table when they welcome Europe-chasing Celta Vigo on Saturday.
Barcelona coach Hansi Flick on Friday demanded to meet with La Liga officials over what he called a "joke" schedule, saying Spanish football was failing to take care of players.
"They made our lives very hard but what was important is we made it to the semi-finals."
Flick's side, undefeated in 24 games across all competitions, are not taking Dortmund for granted despite carrying a 4-0 advantage into the return at Signal Iduna Park.
Barcelona extended their advantage in the Spanish top flight with a 1-0 win at Leganes on Saturday, completing a 12-match unbeaten run and moving seven points clear of second-placed Real Madrid.
Rivals Real Madrid and Barcelona, locked in a title battle in La Liga and set to face off in the Copa del Rey final, have "a lot of respect" for each other, says Los Blancos coach Carlo Ancelotti.
Spain's National Sports Council (CSD) on Thursday upheld an appeal filed by Barcelona players Dani Olmo and Pau Victor which will allow them to play for the club until the end of the season, a decision LaLiga said they would appeal.
But Robert Lewandowski came to the rescue at 1-1 as the 36-year-old netted twice to guide Barcelona to the win. The Polish striker also extended his tally at the top of LaLiga's scoring chart to 25 goals.
Diego Simeone's red-hot Atletico are one of the most in-form teams in Europe on an 11-game winning streak in all competitions and looking like a well-oiled machine with Antoine Griezmann, Julian Alvarez and Rodrigo de Paul firing on all cylinders.
Barcelona's young team can learn from the setbacks they have faced in their LaLiga campaign and continue to improve, manager Hansi Flick said on Tuesday ahead of their Champions League meeting with Borussia Dortmund.
Barcelona coach Hansi Flick on Friday said teenage winger Lamine Yamal will return after three weeks out with injury for this weekend's game against Las Palmas in La Liga.
Barcelona coach Hansi Flick said Friday he would ask his players if they can win without injured teenage winger Lamine Yamal.
Flick's tactical approach, featuring an extremely high defensive line, also seemed ripe for Mbappe and Vinicius to pillage with their pace.
The Catalan giants triumphed at the Santiago Bernabeu to snap Madrid's run of 42 matches unbeaten and continue their own revival under the German coach who arrived this summer.
When Flick was coach of Bayern they thrashed Barcelona 8-2 in the 2020 Champions League quarter-finals.
The 36-year-old hitman looks far sharper than last season and has 12 goals in 11 games across all competitions.
The Catalans won all seven of their opening fixtures but Flick's heavily rotated side came badly undone in Pamplona as Ante Budimir scored a brace either side of Bryan Zaragoza's strike.
Barcelona coach Hansi Flick backed reserve goalkeeper Inaki Pena to replace stricken team-mate Marc-Andre ter Stegen on Tuesday, but also said the club must discuss if they will sign a replacement.
