Borussia Dortmund slumped to a shock 2-1 defeat at Freiburg on Saturday as star striker Erling Braut Haaland drew a rare Bundesliga blank.
It was the first time Dortmund have lost to Freiburg in Germany's top flight for 11 years, with Saturday's setback coming just four days after losing 3-1 to Bayern Munich in the German Super Cup.
"We made stupid mistakes, twice!" grumbled Dortmund centre-back Manuel Akanji.
"We gave away a free-kick for the first and the second goal was a counter attack.
"We can't invite opponents to score, especially when we have Champions League aspirations."
Dortmund will discover their group opponents when the draw for the Champions League is made on Thursday.
Freiburg raced into the lead when dead-ball expert Vincenzo Grifo blasted in a superb free-kick after just six minutes.
Dortmund thrashed Eintracht Frankfurt 5-2 last Saturday on the opening weekend, but the visitors' defensive frailties emerged again having been a problem last season.
After England midfielder Jude Bellingham lost the ball, Freiburg forward Roland Sallai was left unmarked to smash home his shot as the hosts doubled their lead on 53 minutes.
Bellingham made amends by pressuring the defence with a strong run which caused Freiburg midfielder Yannik Keitel to put the ball into his own net with half an hour to go.
Freiburg had been winless in their previous 19 meetings with Dortmund dating back to May 2010.
Bayer Leverkusen later romped to a 4-0 victory at home to Borussia Moenchengladbach.
Visiting goalkeeper Yann Sommer had a nightmare start when Mitchel Bakker's shot hit the post, deflected off the Gladbach keeper and rolled over the line for an own goal.
Czech striker Patrik Schick then doubled Leverkusen's lead on eight minutes.
Gladbach lost striker Marcus Thuram with an injury after just 20 minutes, then their captain Lars Stindl had a penalty saved just before the break.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ