Barcelona fell to a 2-1 defeat at Monaco on Thursday in their Champions League opener, hindered by an early red card shown to Eric Garcia.
Hansi Flick's side have started the season with a perfect record after five La Liga matches but struggled to find their best level against the Ligue 1 side.
After Garcia was sent off for pulling down Takumi Minamino as he ran through, Maghnes Akliouche fired Monaco ahead.
Starlet Lamine Yamal scored a fine equaliser for Barcelona but George Ilenikhena grabbed the winner for Monaco in the 71st minute.
"Winning against Barca, one of the biggest clubs in Europe, makes us happy and proud," said Monaco coach Adi Hutter.
"We played with courage and created a lot of chances."
Five-time winners Barcelona, last crowned European champions in 2015, have struggled in Europe in recent seasons, particularly away from home and were brought crashing down to earth in the French Riviera.
"After 10 minutes with the red card the game changed totally, but what I can see is really the positive thing," Flick told reporters.
"We tried to defend as a team and also attack as a team, and we had chances, but today they deserve the 2-1 (win), so we have to accept that."
Monaco thrashed Barcelona in a pre-season friendly and had the Catalan giants' number for much of the clash at the Stade Louis II.
They were significantly aided by Garcia's dismissal after 11 minutes, for felling former Liverpool midfielder Minamino on the edge of the box.
A careless pass from Barcelona goalkeeper Marc-Andre ter Stegen put Garcia under immediate pressure and he tangled with the Japan international as he tried to stop him running in on goal.
"We didn't understand each other well in that situation and I feel bad for Eric, as in the end it hurts him and we had to play nearly 80 minutes with one less player," Ter Stegen told Movistar.
"Something happened that shouldn't have, to anyone, to me or to him... this defeat hurts because with 10 men we still showed up and tried to take at least a point."
- Inspired Yamal -
Monaco soon took the lead after 17 minutes when Akliouche worked his way into the box and finished well under little pressure.
With Barcelona at a numerical disadvantage the hosts were on top and Raphinha squandered the visitors' best openings despite his impressive domestic form of late.
Monaco had kept Spain's Euro 2024 star Yamal quiet but he burst into life to level before the half-hour mark, cutting in from the right and drilling inside the near post for his first in the competition.
The teenage made it look easy to score a goal out of nothing, becoming the second youngest scorer in Champions League history at 17 years and 68 days old.
Hutter's Monaco took the lead when the dangerous Vanderson played a long ball over the top which substitute Ilenikhena ran on to, with Inigo Martinez in his rearview mirror.
The 18-year-old Nigerian forward hit his shot hard and low and it flew into the net via a hand from Ter Stegen, who might have kept it out.
The German stopper achieved some redemption when he produced a superb save to keep out Folarin Balogun's ferocious effort.
Monaco were awarded a penalty late on when Balogun tumbled to ground under pressure from Martinez, but the referee changed his decision after a VAR review.
Flick threw on Fati for his first Barcelona appearance since August 2023 in search of an equaliser but it was not forthcoming.
"The second goal didn't have much of a build-up and it was hardly deserved in that moment," added Ter Stegen.
"In the end you end up feeling bad about not having taken away any points."