Elsewhere, last season's fourth-placed side Lazio enjoyed a 2-0 away win over Cagliari with goals from Manuel Lazzari and Ciro Immobile.
Ranking fourth in the preliminary heats of the women's 400m individual medley, Ye didn't advance to that final, as she failed to make the top eight in a round of physical tests, a new requirement for this year's championships.
Dortmund responded with furious attacks but Gikiewicz was alert enough to keep out promising chances from Thomas Meunier and Erling Haaland before the break.
Gianluca Caprari pulled one back for Benevento in the 34th minute, before Hakimi got his first goal for his new club in the 42nd minute.
Ronald Koeman saw his reign as Barca coach get off to a flying start on Sunday when Ansu Fati led them to an easy 4-0 win at home to Villarreal, with all four goals coming before the break.
The other two German participants - RB Leipzig and Borussia Monchengladbach - have harder tasks in Groups F and B respectively.
Mendilibar lost several key players, such as Fabio Orellana, Charles and Escalante over the summer, while Ivan Ramis retired, and so far adequate replacements haven't arrived leaving the squad weaker than the one which struggled last season.
Bayern started highly motivated in the encounter and took command while Hoffenheim kept 11 men behind the ball in the opening stages.
In women's singles actions, 23-time Grand Slam champion Serena Williams survived a topsy-turvy opening set to ultimately ease past fellow American Kristie Ahn, 7-6(2), 6-0 to advance. Enditem
Valencia were held 1-1 at home to recently promoted Huesca in a game the visiting side probably deserved to win, but had to settle for Dimitrio Siovas' header cancelling out Daniel Wass' fortunate opener for the home side.
The contract of Karimi with Esteghlal has expired and he is a free agent at the moment. Enditem