USA forward Gio Reyna had the ball in the net in the final minute for Dortmund but the goal was struck off for offside.
Despite qualifying first in a Champions League group including Paris Saint-Germain, AC Milan and Newcastle, Dortmund have now won just one of their past eight league matches.
Dortmund hosted Mainz for the first time since May, when the home side could only manage a 2-2 draw, which allowed Bayern Munich to move past them and claim the Bundesliga title.
Terzic said Monday the collapse “played no role at all” in the side’s preparation for the match and his team started well, overrunning a Mainz side who have won just once all season.
Mainz somehow held on amid an early Dortmund barrage, with English teenager Jamie Bynoe-Gittens hitting the crossbar and blasting wide.
Dortmund eventually broke through after half an hour, Brandt sending a dipping free-kick past Mainz goalkeeper Daniel Batz.
The one-way traffic continued until just before the break, Mainz equalising through Liverpool loanee Van den Berg, who headed in a rebound from a corner.
Dortmund ‘keeper Gregor Kobel got a paw to the header but replays showed the ball was inches over the line, with Mainz scoring their first goal since November.
The home side dominated possession in an increasingly tense second half but could not find a winner, while Mainz carved out a handful of half-chances on the counter.
Dortmund finished the match with 10 men as Sebastien Haller picked up an injury to his ankle after Terzic had made all of his five changes.
Elsewhere, promoted Darmstadt came from behind three times to draw 3-3 at Hoffenheim.
Ihlas Bebou scored two goals and won a penalty which was converted by Andrej Kramaric and looked to have Hoffenheim on course for victory, but Darmstadt’s Tim Skarke got his second with five minutes remaining to rescue a point.
AFP
