Valentino Rossi warned there are not many ideas left to try after another day of 'big frustration' with his M1 in wet conditions, during Friday practice for the Japanese MotoGP.

While the latest in several chassis changes, introduced from Silverstone, has helped improve the rear grip issues in the dry, it's still a case of 'no way' in the wet.

The Italian, competing in his second event since returning from a broken leg, was twelfth fastest. Rossi was a fraction slower than team-mate Maverick Vinales, struggling with the same issues, but 1.5s from Ducati's Andrea Dovizioso.

"Sincerely, we tried to do a lot of things with the bike but we are in trouble," declared Rossi, speaking in the Motegi paddock as rain continued to fall.

"And we don't understand why. Because last year I was very competitive in the wet. I had a good feeling with the old bike. But this year we are struggling. It's something strange.

"We try to modify but at the end we are quite far from the top and the bike is difficult to ride and in general the problem is a bit like in the dry; we don't have rear grip."

Rossi explained that simply reverting to last year's chassis was never an option due to other changes made for 2017.

"I try [the 2016 chassis] at the beginning of the season, but is very difficult to make [the bike] exactly like last year because the engine is different, the seat is different… so in the end maybe I never try!"

Instead Yamaha has introduced a stream of chassis modifications to try and revive last year's rear grip, while retaining the superior end-of-race performance, culminating in the '2018 prototype' used from Silverstone.

"When we race with this [latest] chassis in the dry, like in Silverstone and Aragon, it is not so bad. We have some problem, but we were quite competitive.

"But in the wet, no way. Very difficult.

"If you look at the data with the old bike it is a big frustration. Because f**k in the corners, no way! So we need to try to understand. Especially for next year.

"We are quite worried because today we do a lot of modifications but at the end, no way. So we don’t have a lot more to do than this, this weekend, so we need to understand for next year.

"Already in corner entry I'm very slow. I don’t feel the rear. In general there is always [a lack of rear grip] and in acceleration we have a lot of spin."

The contrast between the difficulties Rossi and Vinales are facing relative to last year's machine was illustrated by the form of fifth-fastest Johann Zarco on the 2016 Tech 3 machine.

But most telling, in Rossi's opinion, was the performance of the two Yamaha test riders…

"Zarco is a good rider in the wet and he is one-second faster than me and Vinales," Rossi said. " But for me, it is interesting to see the two Japanese test riders."

Veteran Katsuyuki Nakasuga, making his usual wild-card entry on a 2017 M1, was 24th and last on Friday (+3.464s). Meanwhile young 'apprentice' Kohta Nozane was 13th (+1.576s) as a last-minute replacement for the sick Jonas Folger, on the older Tech 3 bike.

"Nakasuga is with our [2017] bike and Nozane is with the old bike," Rossi confirmed. "Usually Nakasuga is faster than Nozane and have more experience. But today he is two-seconds slower! So is a big step.

"Entry, exit, we don’t have grip. So we have to struggle with the bike. We try, but we don't fix the problem."