Ex-Formula 1 driver Jenson Button says the SUPER GT cars were only "surviving" as they struggled to adapt to the DTM's wet tyre during the two series' shared Hockenheim weekend.
As the two championships shared a track competitively for the first time last weekend, 2009 F1 champion Button put in the best showing of the SUPER GT contingent on Saturday, maximising the drying conditions to put his Kunimitsu Honda sixth in qualifying.
Button then bounced back from a slow pitstop that dropped him to 16th and crossed the line ninth, but he was only able to muster 16th in Sunday's wet race - despite his team improving its pitstop significantly.
The Honda driver had said on Saturday that he expected wet conditions would hurt the SUPER GT teams as they tried to come to terms with the wet, control Hankook tyre.
SUPER GT has a tyre war in its regular season, but racing alongside the DTM, its teams were forced to used the control Hankook of which the Japanese teams had limited experience.
Asked by Autosport to summarise his weekend, Button said: "Yesterday [Saturday] was fun in the dry.
"I still think there is a lot of work we need to do to actually get the tyres working. But in the wet, it wasn't [fun], yeah.
"To be fair we might as well not call it racing, because we couldn't get any temperature in the tyre.
"We are not competing with DTM, it's just surviving really out there [because of the tyre].
"Yeah, so wasn't really a fun day."
SUPER GT's learning process ahead of the joint-racing at Fuji got off to a difficult start when its Thursday two-hour test was hit with changeable conditions, before Nick Cassidy and Tsugio Matsuda both had offs during practice in their TOM'S Lexus and NISMO-run Nissan respectively.
When Cassidy returned to the wheel on Sunday in the wet finale, he looked set to be the top SUPER GT runner but crashed after a clash with Jonathan Aberdein, while the Nissan also had several reliability issues through the weekend.
When Button was asked why the SUPER GT teams struggled in wet conditions, he said: "With the dry tyres, we can get them in a working window. But the wets, we can't.
"Obviously, it's very difficult for us as drivers, coming and using a tyre so much harder than we're used to.
"You learn to adapt and, I think we did as drivers, but the cars didn't.
"We tried everything, but we couldn't switch the tyres on, and we were running three seconds slower than these [DTM] guys.
"A lot of learning [and], hopefully, for the Fuji race all the guys will learn from what we did today and how bad it was!
"Hopefully [the SUPER GT teams] give these [DTM] guys a bit more of a challenge in Fuji."
Button added that racing alongside the DTM had taught SUPER GT "so much" as the DTM's control tyre forced the Japanese teams into wider set-up experiments for the first time.
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He also expanded on the differences between the DTM and SUPER GT cars, and the areas it gives Honda, Lexus and Nissan to learn from.
"In terms of set-up, we've done more set-up work than the whole SUPER GT season," said Button.
"But also, when you look at the driveability, these guys are running over kerbs and able to get on the power. They get oversteer, but it looks like you can hold onto it. With us, it's a sudden snap.
"There's a lot to look at with driveability of the powerunit, which I think is quite exciting.
"We have so much grip on corner exits that we don't normally feather the throttle, we just plant it and it grips up and we can go. It's very different.
"I think it really helps the SUPER GT teams that are here for the future, racing in Japan. Helps them understand the power and areas we don't really delve into."