Haris Tabakovic scored with two minutes remaining to give Borussia Moenchengladbach a 1-0 home win over Borussia Dortmund on Sunday, while Mainz pushed St Pauli closer to relegation.
Dortmund, who secured Champions League qualification last week, were harmless in attack against a Gladbach side who needed a win to guarantee top-flight football next season.
In the dying moments, Tabakovic broke through and finished past Dortmund goalie Gregor Kobel to make Gladbach mathematically safe from the drop this season.
Despite a third defeat in four games, Dortmund are sure of Champions League football next season and need two points to take second spot behind champions Bayern Munich for the fifth time in the past eight seasons.
"It wasn't good," coach Niko Kovac said to DAZN. "Gladbach were sharp, they had bite – they played like they had a lot at stake, and it looked like we did not.
"The defeat was justified."
Nico Elvedi had a strike ruled out for the hosts midway through the opening half for offside.
Tabakovic, Gladbach's most dangerous attacker, then forced Kobel into a close-range save and fired just off target early in the second period.
With two minutes remaining, Tabakovic put the hosts ahead. His goal, ruled offside by the linesman, was allowed by VAR to give Gladbach their first win over Dortmund since November 2022.
Earlier on Sunday, goals from Phillip Tietz and Phillipp Mwene took Mainz to a 2-1 win at St Pauli, pushing the hosts closer to the relegation mire.
St Pauli lost despite a late consolation through Abdoulie Ceesay and sit in the relegation play-off spot, six points from safety with two games remaining.
Winless since February, Pauli are just one point ahead of second-last Wolfsburg who are in the automatic relegation places. Wolfsburg can leapfrog St Pauli by avoiding defeat at Freiburg later on Sunday.
"We've got no time to dwell on these results," St Pauli captain Jackson Irvine told DAZN.
"We've got two matchdays to fight for our life and we've got to try and put a maximum performance in and see what happens."
St Pauli have spent much of their existence in the second and third tiers but were promoted to the top flight Bundesliga in 2024.
Mainz's victory means Urs Fischer's side cannot be relegated.