The early-season pacesetters, who started the season with seven wins in a row, have won only once at home since that unbeaten start was ended.
And they had to come from behind today to grab a point after Papiss Demba Cisse put the visitors in front in the first minute, Sami Allagui heading the equaliser in the 75th minute.
Cisse fired home with his left foot from eight yards out, arrowing the ball into the top corner.
Mainz quickly responded and came close to an equaliser after four minutes when Eugen Polanski's drilled free-kick brought a superb save from goalkeeper Oliver Baumann.
And Florian Heller missed a good chance to get the home side back on level terms after 12 minutes when he fired over the bar from 12 yards.
Mainz had the first real chance of the second half but Andre Schurrle drilled a right-footed shot over the bar from just outside the area.
However, the home fans were eventually able to celebrate 15 minutes from time when a corner from Andreas Ivanschitz was met by Allagui, who beat everyone in the air to head beyond Baumann.
The visitors never threatened to get back in front and Mainz could have snatched all three points at the death, but Marcel Risse dragged his left-foot shot beyond the post.