Mainz scored either side of the break through Jonathan Burkhardt and Jean-Philippe Mateta to stun hosts Borussia Dortmund 2-0 on Wednesday and take a big step towards remaining in the Bundesliga for another season.
With Bayern Munich having secured their eighth consecutive league title on Tuesday and Dortmund certain of a top-four finish and a Champions League spot, the hosts rarely threatened in the first half.
Although Dortmund club bosses had warned that the season was not finished despite having secured a Champions League spot, they looked completely unmotivated.
Dortmund made feeble attempts to fight back but they were lucky not to concede a third goal as Mainz created several good chances.
Mainz are now 15th on 34 points, five above the relegation playoff spot with two games remaining.
Dortmund remained in second place on 66 points, 10 behind Bayern, after RB Leipzig, in third on 63, conceded two late goals to draw 2-2 with Fortuna Dusseldorf. Leipzig host Dortmund on Saturday.