Koln beat Bayer Leverkusen for the first time since the 2015/2016 season with a 1-0 victory away from home.
Leverkusen opted to rest several key players with Thursday's crucial Europa League last 16 clash against Atalanta in mind and that came back to bite them.
Koln were coming off the back of a 1-0 home defeat to Hoffenheim last time out and broke a two-match winless streak with the victory.
Kingsley Schilder netted the only goal of the match in the 67th minute to move into seventh position on the standings, while Leverkusen remain in third by just a point.
Borussia Dortmund cut Bayern Munich's lead atop the Bundesliga standings with a 1-0 win over Arminia Bielefeld.
Bayern drew 1-1 with Hoffenheim on Saturday, allowing Dortmund to cut the Bavarians' lead to seven points, while Dortmund have a game in hand with the sides still to meet.
Marius Wolf scored the only goal of the game in the 81st minute as Marco Rose's side left it late.
Meanwhile, Arminia fell to their third successive defeat and are now just two points above the relegation zone.
Eintracht Frankfurt edged closer to the top eight with a 2-1 victory over VfL Bochum.
Only two sides separated the sides prior to kickoff, with Frankfurt the higher of the two on the table.
Both sides were coming off the back of victories. For Frankfurt, it was a morale-boosting win away at La Liga outfit Real Betis in Europa League action on Thursday, while Bochum beat Hertha Berlin 4-0 last weekend.
It appeared to be going downhill for Frankfurt when Sebastian Polter gave Bochum the lead in the 19th minute. However, Oliver Glasner's side rallied back through an own goal from Erhan Masovic and a Daichi Kamida strike within six minutes of each other shortly after half-time.
Frankfurt are in ninth, one point off the top eight, while Bochum are in 11th and losing ground.