Cologne will have the chance to escape relegation in the final Bundesliga matchday next weekend after scoring a goal in stoppage-time to secure a 3-2 comeback win against Union Berlin on Saturday.
Mainz, meanwhile, cruised to a 3-0 victory against Champions League finalists Borussia Dortmund, who rested several starters.
With 32 points, Mainz are no longer in danger of automatic relegation and will remain in the Bundesliga if they defeat VfL Wolfsburg next weekend. Other results could lead to a relegation play-off against the third-placed in the second division.
Union sit two points behind, currently in the play-off spot, and host Freiburg next week. They have a three-point advantage over Cologne, who visit Heidenheim and can't leave the drop zone any more. Their only chance of survival is securing a ticket to the play-off.
Cologne needed a win to avoid automatic relegation this weekend, but the mission became even more difficult after Robin Knoche and Kevin Volland from the spot gave Union a 2-0 lead with less than 20 minutes played.
They pulled one back shortly before the break, but the atmosphere at their Rhein Energie stadium was dim as relegation to the second division seemed just a matter of minutes.
That changed completely once Steffen Tigges found the equalizer in the 87th minute and when Damion Downs secured Cologne a crucial win in the third minute of stoppage-time.
"It can't be put into words. I don't think it has ever been this loud in the stadium. It wasn't the most beautiful football, but we left our hearts on the pitch. We'll go home with a smile today and prepare for the final against Heidenheim," Cologne defender Timo Hübers said.
Mainz faced a Dortmund side that made 10 changes in the starting line up after their mid-week Champions League win against Paris Saint-Germain.
The hosts took the lead in the 12th minute from Leandro Barreiro's side-foot finish. Jae-sung Lee made it 2-0 just six minutes later before he completed a brace in the 23rd.
"They were always one step ahead. I had imagined a different last away game," said Dortmund forward Marco Reus, who's leaving the club at the end of the season after a total of more than 21 years of service.
Elsewhere, Freiburg played out a 1-1 draw with Heidenheim in coach's Christian Streich final home game with the team. The draw was enough to take Freiburg closer to European qualification.
The 1-1 score also prevailed in matches between RB Leipzig and Werder Bremen, and Borussia Mönchengladbach and Eintracht Frankfurt.
Streich said ahead of the game that he wasn't letting his emotions show for now out of self-protection.
"I've built myself a little box, inside me, and everything goes in there. Otherwise I won't make it. Otherwise it's up and down, then you're emotionally exhausted, it's too much," he told broadcasters Sky.
Gladbach, meanwhile, confirmed their Bundesliga status for another year and Frankfurt guaranteed they will finish sixth, which currently means qualification for the Europa League. But the sixth place could become a Champions League spot if fifth-placed Dortmund are crowned Champions League winners this season.
