Two goals from Jae-sung Lee took Mainz to a shock 2-1 home win over Bayern Munich on Saturday, handing coach Vincent Kompany his first league loss.
Lee scored in each half against an uncharacteristically poor Bayern, who are now just four points clear after Bayer Leverkusen's 2-0 win at Augsburg.
Injuries meant Kompany was unable to call on several first team regulars including Harry Kane, Manuel Neuer, Alphonso Davies, Kingsley Coman, Serge Gnabry and Joao Palhinha.
Kane's absence was particularly telling in the first half, with the league leaders dominating possession but unable to convert any of their half chances into a goal.
Mainz boasted a better-than-expected home record against the German giants, having won three of their past four home league games over Bayern.
The home side struck shortly before half-time, Lee taking advantage of a poor clearance from his South Korea teammate Min-Jae Kim to hammer home.
Even with the knowledge that Leverkusen were ahead in Augsburg, Bayern came out poorer after half-time, lacking precision and energy.
Mainz soon capitalised through Lee again. Armindo Sieb threaded a clever cross goalwards for Lee to collect before blasting home on the spin.
Leroy Sane tapped in a deflected Joshua Kimmich shot in the 87th minute to give the league leaders hope, but Mainz held firm for a famous home win.
The win moved Mainz, who battled relegation last season, up to sixth.
Defending champions Leverkusen continued their recent resurgence, winning comfortably 2-0 at Augsburg, their seventh victory in a row.
Martin Terrier put Leverkusen in front 14 minutes in, curling the ball home with his first touch after a lovely low cross from Jeremie Frimpong.
Florian Wirtz added a second in the 40th minute when he controlled the ball, sold the Augsburg defence a dummy before skipping a shot across the grass and into the bottom corner.
A double from Alassane Plea and goals to Robin Hack and Tim Kleindienst took Borussia Moenchengladbach to a 4-1 win over Holstein Kiel.
Gladbach have now lost just one of their past eight league games while Kiel, playing their first season in the top flight, sit second last.
In Saturday's late game, promoted St Pauli host Werder Bremen, while Champions League participants Borussia Dortmund, Stuttgart and RB Leipzig are all in action on Sunday.