Dortmund entered their trip to Mainz in high spirits after a 2-0 UEFA Champions League win over Arsenal on Tuesday.

But they lost their second Bundesliga match of the season " just four games old " after Shinji Okazaki gave Mainz a 66th-minute lead.

Dortmund substitute Ciro Immobile then missed a penalty before a Mathias Ginter own goal capped a miserable day for Jurgen Klopp's side.

And Bender " who made his first Bundesliga start since February after a serious groin injury " said his side were wasteful in the defeat.

"We were not ambitious enough," Bender told Dortmund's official website.

"We were aware that it would be a very tough match. [But] we have shown that we know how it goes against Arsenal.

"We must remain clear in our actions [and be] purposeful.

"After the break we had the game under control but one counterattack... [and] the game [is turned] on its head.

"We need to be wide awake on Wednesday against Stuttgart."