Haaland hits hat-trick as Dortmund lose, Bayern crash at Mainz
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In Dortmund, Bochum raced into a 2-0 lead after just eight minutes thanks to goals by Sebastian Polter and Gerrit Holtmann.
Haaland led the fight back by converting two penalties, then tapping in his third goal with an hour played.
However, Bochum hit back with goals in the final ten minutes.
Juergen Locadia fired one in before Milos Pantovic converted the winning penalty after a handball by Dortmund midfielder Axel Witsel.
Bochum claimed their second significant scalp this year following a shock 4-2 home win over Bayern in February.
Bayern suffered a hangover from last weekend's title party as hosts Mainz also hit the woodwork four times in a 3-1 win.
Goals by Germany Under-21 striker Jonathan Burkardt and French defender Moussa Niakhate gave Mainz an early 2-0 lead.
The Bavarians pulled one back when Robert Lewandowski rolled in his 34th Bundesliga goal this season, claiming a league record after scoring 18 times away this term.
Midfielder Leandro Barreiro grabbed Mainz's third goal just after the break when his shot took a wicked deflection and looped into the Bayern goal.