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. Bayern came out poorer after half-time, lacking precision and energy. Mainz soon capitalised through Lee again. Leroy Sane scored, but Mainz held firm for a famous home win.