Bundesliga leaders Bayern Munich suffered a first league defeat under boss Vincent Kompany as they were beaten at Mainz.
South Korea forward Lee Jae-sung took advantage of a defensive mistake to give Mainz a 41st-minute lead and added a second with a fine turn and finish on the hour mark.
Leroy Sane pulled one back three minutes before time for Bayern, who were missing several players through injury including England striker Harry Kane.
"We played against a team with a lot of life and spirit and they were prepared to fight for every moment," said Kompany.
"From our side there was not a lack of fighting. We did that to the best of our ability. Today we were not at our best.
"This game, we can't play it any more. It's done. We'll use it to build a fire for next week."
Defending champions Bayer Leverkusen closed the gap to four points with a 2-0 win at Augsburg thanks to goals from Martin Terrier and Manchester City target Florian Wirtz.
