Vinales left it until the closing stages of Thursday's running to become the first rider of the Sepang test to dip beneath the 1m59s barrier, outpacing nearest rival Alex Rins by more than half a second.

His laptime of 1m58.897s was just shy of the unofficial track record set by then-Ducati rider Jorge Lorenzo this time a year ago in pre-season testing.

Rossi meanwhile was sixth-fastest, 0.728s off the pace set by Vinales.

"Obviously I used a very old tyre that did not allow me to make the laptime I could have done," said Rossi. "I think I could have done better, maybe a tenth, I would have been in the top three.

"It's a shame, because everyone [ahead of me in the standings] did a time attack, but Maverick was unattainable. I could never have done a similar laptime.

"But anyway, we are not doing the races today but trying to find a good feeling."

Rossi confirmed on Wednesday that Yamaha had chosen to only bring one type of engine to Sepang, and said Thursday's running was spent honing the M1's engine braking and electronics, as well sampling a new type of chassis aimed at improving rear grip.

But while satisfied with the overall direction, Rossi emphasised he doesn't feel the improvements are yet sufficient to put the team back on an even keel with Honda and Ducati.

"In my opinion we are now a little closer to the others, but it is still not enough," added the Italian. "Today Ducati has been very strong; I don't know what tyres they were using but they were very fast.

"It seems we are still missing something but we are moving in the right direction, by a little."

Vinales: Yamaha has found old DNA again

After his chart-topping time, Vinales said he felt it was important that Yamaha "keeps its feet on the ground", pointing out the fact he has yet to attempt a full race simulation during this test.

But the Spaniard was positive about the 2019-spec M1, which he feels has the 'DNA' of old Yamaha machines in terms of its performance in high-speed corners.

"The direction we have taken with the engine brake is a bit the one I asked for," said Vinales. "Now I'm better on the corner entry, the engine and the electronics have improved a lot.

"We still have to improve according to the details, but I'm very happy with the work Yamaha has done this winter, I can push and ride my way.

"If we get a bit more smoothness in the power delivery it would be very positive. In the high-speed, flowing sectors the bike behaves very well, I think it has recovered the DNA of the Yamaha."

He added: "We have to improve with a full tank [of fuel] and do 20 [consecutive] laps. We need to know how we're doing when we don't have grip; that's why we're trying to simulate these conditions.

"Tomorrow we'll do a race simulation at 3 o'clock in the afternoon."

Additional reporting by Oriol Puigdemont