Youngster Mario Goetze opened the scoring on 26 minutes after a fine individual effort.

The 18-year-old German U-21 midfielder eluded two challengers inside the area before wrong-footing keeper Christian Wetklo with a low shot.

Mainz missed the chance to equalize when a penalty by Eugen Polanski was saved by Dortmund keeper Roman Weidenfeller shortly after the restart.

Goetze set up Paraguay striker Lucas Barrios for Dortmund's second goal in the 67th minute.

Egypt striker Zidan is still missing as he recovers from a serious knee injury he sustained last April.

"We applied our game plan very well against a tough opponent," Dortmund manager Jurgen Klopp, an ex-Mainz boss, said after the game.

"The saved penalty was a key moment. We had luck on our side there. As for the Bundesliga title, we are giving it exactly zero thoughts at the moment."

Dortmund, who last won the Bundesliga in 2002, have 25 points from 10 games, one ahead of second-placed Mainz.