Two-time Nurburgring 24 Hours winner Mies was signed by the Hitotsuyama squad to share its Yokohama-shod Audi R8 LMS Evo with Shintaro Kawabata.
The German driver set the pace in the GT300 class on the first day at Okayama, a track he had never previously driven at, with a time of 1m25.649s - which proved to be the second-quickest time of the test, beaten only by the Cars Dream Tokai28 Lotus Evora MC.
Commenting on his experience, Mies praised the quality of the Yokohama tyres for their grip in comparison to the spec Pirelli rubber used in GT World Challenge (formerly Blancpain GT Series), the Intercontinental GT Challenge and ADAC GT Masters.
"I really like to finally have a very good tyre," said the 30-year-old. "In Europe, we have one brand which is not very good but everyone has to use it.
"Now to have finally a proper tyre again, it’s a good feeling, good grip. I need some more experience with the tyres, so I’m looking forward to doing more laps on them."
Hitosuyama has endured three miserable seasons in SUPER GT's GT300 division, the team's last race win coming back in 2016 at Motegi.
Mies revealed one of his targets for the year would be to end that losing streak.
"You never expect [to be fastest]," he said. "You always hope for it, but if you’re new to a team, car, tyre and track, you just try and do as good a job as possible and you don’t expect P1.
"I’m really happy also for the team. I think their last win is a long time ago, 2016, so a big goal this year is to have a win with them."
On how he's adapting to the unique challenges of Japanese motorsport, Mies added: "It’s nice but it’s different. It’s more complicated because of the language barrier, but the team is helping me a lot and there’s a translator with me. It’s a lot better than I thought.
"I’m really happy to be here. Everyone knows SUPER GT in Europe and a lot of drivers want to drive here. But it’s very difficult to come here. I’m really enjoying it, I really like Japan."