Confident that Yamaha is back on track with its MotoGP chassis, Valentino Rossi feels acceleration is where the factory is now losing ground - especially to Ducati.

Speaking at the end of day one at the Thailand test, the Italian - celebrating his 39th birthday - explained that Yamaha has dropped behind since the switch to MotoGP's unified ECU software in 2016.

"For me the bike is better to ride [with the 2016 chassis]. Compared to last year I feel better. The feeling is more similar to two years ago and this is very important because I suffered a lot last year, maybe more than Maverick, with the 2017 chassis," explained Rossi, who was the top Yamaha rider in eighth (+0.392s) during MotoGP's debut at the Buriram circuit.

"But for me at this moment all the Ducatis are better in acceleration, because they are more in front compared to us in the electronic. When we moved to the Magneti-Marelli [single software] for me we have some problem. For us it's difficult to understand.

"Honda and Ducati understand something we don't [with the single electronics]. So I hope that we can recover as soon as possible because at this moment we suffer.

"The good thing with the electronics is that it is not like fixing an engine, chassis or swingarm. You can fix it with a number, so you can fix it in a short time," Rossi added.

"But I'm quite worried because if we don't fix it now, I'm not optimistic to fix it by the first races..."

While all teams must now use the same ECU software, they are free to change the target parameters of the system to try and extract maximum performance from their bike.

As MotoGP Technical Director Corrado Cecchinelli previously explained to Crash.net:

"Example: You have to input your target wheelspin, given the lean angle. Everybody has the same strategy that aims to deliver their target spin at a given lean angle, but the value of that target spin is up to you.

"That is where a team can be different from the others. Not by the way in which the electronic strategy acts, but by the target inputs.

"For instance, you may input 10% of spin at 40-degrees of lean. And another team may want 12% at the same lean angle.

"The way the strategy would handle those two different inputs, 10% and 12%, would be exactly the same. Only the input value would be different."

Rossi made clear that the quest to improve the electronics is not related to the low-grip issues suffered last season, which eventually resulted in the removal of the 2017 chassis at the Valencia finale and a return to the 2016 design for this year.

"They are two different things," he said.

"Electronics, especially Ducati and Honda improved a lot during last season and in the second part were stronger.

"Another thing is the chassis. Now [with the 2016 chassis] I brake and enter, the bike turns, I feel good. But we need more acceleration.

"Also, in this track, more than Malaysia, we are very close. Because I am only three tenths from the top but I am eighth and I think that we need to recover under this point of view."

Rossi had previously branded the Buriram circuit as boring, based on a PR visit in 2015, but revised his opinion after 74 laps on a MotoGP bike.

"I rode the track in 2015 with a Yamaha 300 together with Lorenzo and Espargaro. I remember that the track was similar to Austria. But in reality it's better, has some good corners. Technically it's quite easy, but it's not boring.

"The track is in a good condition. It's clean and the asphalt has good grip. This is very important. And also the layout. I remembered it was more similar to Austria, so I was very worried. But when you ride maybe it is more similar to Argentina.

"It's good to ride, you have a good feeling, you enjoy. The track is not very difficult but anyway it's fun."

Turning to the subject of his 39th birthday, Rossi was asked if racing at such an age is in itself a form of challenge or motivation:

"The motivation is to try to go fast, ride in front and fight for the victory. Not age!"

Honda's Cal Crutchlow was quickest on day one of the Thailand test, with Rossi's team-mate Maverick Vinales eleventh on the timesheets.

Testing continues on Saturday and Sunday.