Bundesliga runners-up Bayer Leverkusen started the new season with a 2-0 loss to Mainz thanks mainly to two mistakes from the Leverkusen defence.
A first-half blunder by stand-in goalkeeper Fabian Giefer and a late own-goal from defender Omer Toprak earned the hosts all three points and last season's runners-up were left nursing defeat after last week's shock DFB-Pokal exit.
"It was our fourth game in ten days and in two of those games we had to go over 120 minutes, so what my team did today was top," said Mainz coach Thomas Tuchel after his side had back-to-back UEFA Europa League and DFB-Pokal matches. "We had accepted defeats may come, so I am pleased we managed to avoid that."
But Bayer counterpart Robin Dutt was left fuming. "I am very disappointed," Dutt said. "We lacked accuracy up front, yet we wasted chances to take the lead twice."
Mainz opened the scoring on 32 minutes after defender Stefan Reinartz back-passed to Leverkusen goalkeeper Giefer who completely mistimed his clearance and the ball rolled out of his control.
It turned into a foot race between him and Mainz striker Sami Allagui, which the Tunisia international won and smashed his shot home from a tight angle.
Dutt had dropped American goalkeeper David Yelldell in favour of Giefer after Leverkusen's shock 4-3 DFB-Pokal defeat at Dynamo Dresden the week before with first-choice Rene Adler out with a knee injury.
Mainz made sure of the points when Toprak's sliding tackle led to the ball rolling past the despairing Giefer and into the empty net on 86 minutes.