Robert Lewandowski struck twice for Hansi Flick's Barcelona to deliver a battling 2-1 win at Valencia in their La Liga opener on Saturday.

The German coach celebrated his first competitive match at the helm with three points as Barca came from behind against Ruben Baraja's solid Valencia side.

Hugo Duro sent the hosts ahead after 44 minutes but veteran Polish striker Lewandowski levelled from close range before the break and netted a penalty early in the second half.

With champions Real Madrid, led by new superstar Kylian Mbappe, in action on Sunday at Real Mallorca, Barcelona started their bid to steal the crown with an impressive result at Valencia's rowdy Mestalla stadium.

Barca were without key players, including Frenkie de Jong, Gavi and Ronald Araujo and Flick selected three 17-year-olds in his starting line-up.

They were Spain's Euro 2024 star Lamine Yamal, whose father was stabbed earlier this week in a violent attack, Pau Cubarsi and midfielder Marc Bernal, the latter making his debut.

"In the second half I think we played much better and with three or four very young players -- I think this match is very important, to win at the beginning of the season, especially at this stadium in Valencia, it's very difficult," Lewandowski told Movistar.

"My aim is always to score goals, I don't know how many goals by the end of the season, but it is very important for me that as today was the first game, (I got) goals for confidence."

Barcelona's summer signing Dani Olmo is not yet registered to play and Flick said the playmaker still needs to get up to speed in terms of fitness.

The coach was happier with his team's second half display than their performance before the break.

"The second half was much better than the first half (where) we made a lot of mistakes," Flick told Movistar.

"We came back at the end of the first half and in the second half, we did much better."

An even first half burst into life as the break beckoned after the teams managed to create few clear chances in the first 40 minutes.

Duro nodded the best of them over the bar from a corner, while Valencia goalkeeper Giorgi Mamardashvili saved a fierce Raphinha drive.

Valencia took the lead in the 44th minute, with Duro nodding home Diego Lopez's cross after losing Inigo Martinez, who played him onside.

- 'Under control' -

Los Che should have doubled their lead but Cubarsi hacked Duro's effort to safety off the line after Marc-Andre ter Stegen gave the ball away.

Lewandowski grabbed the leveller in stoppage time, turning home from close range after Yamal diverted Alejandro Balde's cross back into the danger area.

Barcelona moved ahead soon after the break when Valencia defender Cristhian Mosquera felled Raphinha in the area.

Lewandowski, who turns 36 next week, hammered the resulting penalty into the top corner with aplomb.

Barcelona kept Valencia at arm's length for most of the second half, ensuring the pace of the game was slow.

The Catalans had the better chances to score again with Ferran Torres sending a weak effort straight at Mamardashvili when through, while midfielder Marc Casado fired wide when well placed late on.

Lewandowski had a chance to complete his hat-trick in stoppage time but rifled over the bar.

"In the final stages we had this match under control," added the former Bayern Munich striker, who played under Flick in Germany.

"We took the first step on the first match day, and now we have three games (coming up in La Liga) that are very important."

Yamal was applauded after being substituted by some Valencia fans for his performances with Spain as they won the Euros.

"Lamine is a great player and I think everyone loves to see him playing football... I think you can say all the world likes to see him play," added Flick.

Barcelona host Copa del Rey winners Athletic Bilbao next Saturday in their next league match.