'The problem is we don’t know why' - those were the words Valentino Rossi after a better than expected opening day for Yamaha at the inaugural Thai MotoGP.

Rossi and team-mate Maverick Vinales arrived at Buriram braced for a tough event, having struggled badly last time at Aragon, been ranked eighth (Vinales) and twelfth (Rossi) at the pre-season Thai test and fearing the revised (harder) rear tyres would magnify their ongoing grip problems.

Instead, Vinales and Rossi were first and second in FP1 - after fitting new tyres at the end of the session - then second (Vinales) and ninth (Rossi) in the afternoon. But lap times were very close and Rossi was only 0.3s from the fastest lap of the day, by Ducati's Andrea Dovizioso

"The bike is the same, but today we suffer less, so it's better," began Rossi. "For some reason we have a lot more difference between our good and bad tracks. The other bikes are more balanced, always the same.

"Aragon was a disaster because we didn’t have any grip. Here, on paper, we have to suffer, but today less than what we expect. So we are a bit more competitive.

"FP1 was fantastic because we were first and second, but also because a lot of top riders didn't put the new tyre. So I think that this is not the reality, but anyway we can stay in the top ten this afternoon for try to go directly into the Qualifying 2 and try to make a better result compared to Aragon.

"I know that Maverick made some bigger modifications to the balance of his bike. Me, personally, we worked more normally. I don’t know if it's because of [the modifications], but Maverick today is fast – this morning, this afternoon. Especially this afternoon when I think that 95% of the riders made a time attack. He's second and have also a good pace."

But the biggest worry for Yamaha, experiencing its longest win drought in MotoGP, is that the factory still doesn't fully understand why they do better at some tracks and struggle badly at others.

"I don’t know... We don’t know," Rossi said. "You go on the track [here] and it looks like the bike has more grip, so you can push more. Aragon was very difficult, but why? I don’t know.

"The problem is that we don’t know! Also Yamaha don’t know!"

Rossi, who also tried a new Yamaha fairing, added that he wasn't surprised by the change in Michelin tyres relative to the test.

"I expected this modification by Michelin because in the test no tyre can make 25 laps. Not just us, for everybody. The lap time was a lot faster, but the tyre after 10-12 laps started to lose parts. So it was not possible to race with those tyres.

"So, like happens in other tracks where it's very demanding for the rear, like Austria, they bring a tyre that is more hard. So you have less performance, it's a bit more difficult to ride, but can make all the race. I think it's the right choice, also for the safety."

The nine-time world champion also said he's happy to have another race in SE Asia.

"Here the passion for MotoGP is very high, they have money and it's something new. So it's good that we come here more times. Anyway, the major part [of the season] is still in Europe. But I like to race here. Also a little bit less people in the paddock! So more easy!"