Leon Goretzka scored the only goal of the match as Marco Giampaolo's first game in charge of AC Milan resulted in a 1-0 loss to Bayern Munich at the International Champions Cup.
Giampaolo arrived from Serie A rivals Sampdoria after Gennaro Gattuso stepped down as head coach of Milan at the end of the 2018-19 season.
And Giampaolo oversaw defeat in Milan's pre-season opener as Goretzka's well-taken 48th-minute goal guided Bayern to their second ICC victory.
New signing Theo Hernandez made his debut for Milan following his transfer from Real Madrid, while Paolo Maldini's son – Daniel – was also in the starting XI for the Rossoneri.
Niko Kovac's Bayern controlled proceedings in Kansas City, where clear-cut chances were few and far between in a tight opening 45 minutes.
Maldini's side-footed effort sailed just over the crossbar, while Bayern midfielder Renato Sanches fired a powerful strike just past the post.
Hernandez's bow ended prior to half-time as the French full-back was carried off on a stretcher with an ankle injury.
Bayern managed to break the deadlock right on the stroke of half-time when Joshua Kimmich played a perfectly weighed pass to Goretzka, who took a touch before tucking the ball beyond Gianluigi Donnarumma in the 48th minute.
Patrick Cutrone – who has been linked to Premier League side Wolves – should have equalised for Milan after cutting out Niklas Sule's pass across goal but the second-half substitute was denied by Sven Ulreich in a one-on-one situation with 26 minutes remaining.
Bayern found the back of the net nine minutes from the end but Jann-Fiete Arp's goal was ruled out for what appeared to be a handball, while David Alaba had an effort cancelled out for offside.