Borussia Dortmund missed a golden chance to go top of the Bundesliga on Saturday when they surrendered the lead in a "bitter" 3-2 defeat at Cologne.
Leaders Union Berlin lost at Frankfurt but Dortmund failed to capitalise just a week ahead of their heavyweight clash with defending champions Bayern Munich.
They led Cologne 1-0 at the break before conceding two costly goals in two minutes early in the second half to leave manager Edin Terzic lamenting his fragile side's third Bundesliga defeat.
"It's very disappointing for us - it's very bitter," said Terzic.
"We weren't ready to show our toughness (in the second half).
"At one of the most important stages in the game, we were not ready to defend our goal."
Cologne manager Steffen Baumgart - whose side have lost only once in the Bundesliga this year - praised his team for "a very good performance".
Dortmund led at halftime thanks to a Julian Brandt goal, set up by English teenager Jude Bellingham, who was named captain for the first time due to Mats Hummels' late withdrawal.
"We should have done ourselves a favour - we have to do more," said a disappointed Brandt.
The visitors dominated the opening half and looked in full control of the game, until Florian Kainz equalised in the 53rd minute.
Former Dortmund striker Steffen Tigges took advantage of poor defending from a Dortmund corner just two minutes later, climbing above the defence to head home.
A suddenly disorganised Dortmund continued to defend poorly, giving midfielder Dejan Ljubicic far too much time in the 71st minute, allowing the midfielder to guide in a long-distance shot to put the game beyond doubt.
Dortmund pulled one goal back through a lucky deflection in the 78th minute, but Cologne held on for a deserved victory.
In Saturday's late game, Werder Bremen thrashed Borussia Moenchengladbach 5-1 at home, thanks to two goals from Niclas Fuellkrug, who now sits atop the Bundesliga scoring charts with seven goals in eight appearances.
While Gladbach, who had conceded just five times in seven games leading into the game, could have edged Dortmund out of fourth place with a victory, Bremen exploded out of the gates and led 3-0 after just 13 minutes.
Bremen added a fourth just before halftime and another in the second half as they secured their first win at home since being promoted from the second division.
After the match, Bremen fans chanted for Fuellkrug to "fill Germany's hole" up front in the coming World Cup, while the man himself credited his "world-class" team for his impressive form.
"I'm in good form and I want to stay in good form, but it's always the team who contributes to that."
FIRST LOSS FOR UNION
Elsewhere, an early goal from Mario Goetze followed by an individual effort from Jesper Lindstroem guided Frankfurt to a comfortable 2-0 win over Union at home.
Frankfurt lost striker Randal Kolo Muani to a second yellow card in the 68th minute, but the home side held firm to beat the previously unbeaten Berliners.
Berlin coach Urs Fischer said after the game "we did not bring the basics which usually characterise us onto the pitch".
The season's other surprise package alongside Union, Freiburg, moved up to second after a dominant 2-1 win at home against Mainz.
Freiburg sprinted to a 2-0 halftime lead thanks to goals from Michael Gregoritsch and Daniel-Kofi Kyereh, but should have been further ahead and endured some nervy moments late after Mainz's Aaron Caricol scored in the second half.
A brace from Timo Werner and a penalty from Christopher Nkunku helped Leipzig to a 4-0 win over struggling Bochum, who are still winless after eight games.
In Wolfsburg, an injury-time goal from Yannick Gerhardt helped the home side to a 3-2 win over Stuttgart, just their second victory of the season.