A win at Wolfsburg next Saturday would secure Mainz's Bundesliga safety
Just days after securing a spot in the Champions League final, Borussia Dortmund were thrashed 3-0 against Mainz at Mewa Arena.
Leandro Barreiro gave relegation-threatened Mainz the lead after just 12 minutes.
Dortmund goalkeeper Alexander Meyer's dreadful pass gifted the ball to Lee Jae-sung and he doubled Mainz's advantage after 19 minutes.
The South Korean scored his second four minutes later after more lacklustre defending from Edin Terzic's side.
After producing a fantastic performance to beat Paris St-Germain and reach the Champions League final on Tuesday, Terzic left a number of his key players at home with fifth place in the Bundesliga secured.
Marco Reus, who announced he will leave Dortmund at the end of the season after 12 years at the club, started but failed to influence the match.
Mainz's win takes them up to 15th, two points clear of Union Berlin in the relegation play-off spot.
Dortmund host Darmstadt at the Signal Iduna Park next Saturday in their final Bundesliga game of the season before facing Real Madrid at Wembley Stadium on Saturday, June 1.
Meanwhile, the Bundesliga will feature Holstein Kiel next season, after they reached the top flight for the first time in their history with a 1-1 draw at home to Fortuna Dusseldorf on Saturday.
Striker Benedikt Pichler gave Kiel the lead inside two minutes but Dusseldorf levelled through Christos Tzolis' 70th-minute penalty.
However a point was enough to secure a top-two finish for Kiel, who were playing in the Regionalliga North - the fourth tier of German football - a decade ago.
