Valentino Rossi's Sepang optimism took a blow as he concluded a difficult Thailand MotoGP test with the twelfth fastest lap time.

While Johann Zarco put his satellite Tech 3 machine at the head of the standings for almost the entire day, Rossi and factory team-mate Maverick Vinales languished at the tail of the top ten.

No-one could doubt their efforts, the pair riding right up to the 5pm closing time. But Rossi was left 0.730s from fastest man Dani Pedrosa, while Vinales was +0.809s and didn't beat his day two time.

So what was the main problem today?

"We always suffer with the rear tyre and we are correlated to the rear tyre," replied Rossi, who celebrated his 39th birthday on Friday.

"For me the problem is that if we are able to go softer, you can go faster. But unfortunately I lose a part of the tyre, chunking, because the temperature of the tyre is too high.

"And after you have to go with the tyre that is a bit more hard [the Austria spec]. When the tyre is a bit more hard, you can arrive at the end of the race but you go more slow! So we worked a lot, but everything is correlated to the rear grip."

Zarco set his best lap time on a medium front-soft rear tyre combination.

"For me we speak a lot about the chassis, we speak a lot about the electronics and everything. But depends very much from the tyre. Because sometimes you change tyre and change all the feeling.

"Very similar to last year and like this is also very difficult to develop the bike, because sometimes you have a problem and then after two hours it disappears."

Nonetheless, Rossi reiterated that Yamaha also has plenty of work to do.

Earlier in the test the nine-time world champion had spoken of the need to try and improve acceleration through progress with the electronics, an area where he feels Yamaha has fallen behind since the unified software in 2016.

"We don't improve a lot compared to last year, especially the electronics. So I think we have a lot of work and at this moment we have to struggle. But this is today. Maybe Qatar can be better.

"I spoke a lot with all the Japanese engineers and I tried to give all my feedback. I think the problem can be quite clear. But not easy to fix. Difficult. For me in Qatar we need to wait to understand our potential on that track. But I don't think we will have something new. We have to think more about the future.

"Apart from that a lot of people are very competitive here. We need to concentrate and try the maximum."

Rossi and Yamaha weren't alone in seeing their form drop at the second of the three official tests and he cited Honda as the most consistent performer so far.

"I was sincerely more happy after Malaysia because I feel better. My pace was better. Here we are struggling more," he explained.

"I think, from what I understand, the only manufacturer that was competitive in both tracks was Honda, especially with Marquez but also Dani and Crutchlow.

"Ducati were flying in Sepang, Lorenzo was first, and here is struggling a lot. So we have to wait for Qatar to see what happens."

Rossi also clarified his chassis specification.

"It's not exactly the 2016 chassis, but the bike of this year is more similar. I feel better because I can enter the corners faster and I feel better about the front. But we still have to work, especially in acceleration."

The Doctor also batted away speculation that he is now dominating development of the Yamaha at the expense of Vinales.

"No. If Maverick wants to use some other stuff, he can. Because also last year sometimes we tried different things."

The final pre-season test takes place in Qatar from March 1-3.