Valentino Rossi believes "a very tough fight" lies ahead after he witnessed six of the top riders lapping the Sepang track within 0.26s of Maverick Vi?ales' fastest time on Wednesday.

The Italian set the fifth fastest time on Wednesday - sixth on the combined timesheets from all three days - thanks to a 1m 59.5s lap, an effort he described as "not so bad" at the close of a shakedown conducted in grueling Malaysian heat.

Rossi had a host of new parts to test across the three days, including a new chassis and revamped fairing, which, for the most part, bore positive results.

It was when he cast an eye over the pace of other riders however - notably Maverick Vi?ales and those riding for Repsol Honda and the factory Ducati team - that he sounded a word of caution.

"At the end the test is positive, because my lap time was good," said a tired Rossi, who was suffering the painful effects of a headache throughout Monday.

"My final lap of 1m 59.5s is not so bad. We are all very close. But especially we find a lot of good things for the bike. Especially this afternoon, we worked more on the new chassis, and I feel good. At the end we found something interesting to improve.

"It was a difficult test because it was very long but we found something positive. All the new stuff - not all, but the major part - is good.

"The problem is that the first riders are all very close and all very fast. Together on the time attack but also on the pace. So I think that it will be a very tough fight."

Asked to specify the positive points of Yamaha's new chassis, Rossi only offered a brief, "In general," while he was also muted on the factory's new fairing, which features internal ducts that may be designed to increase downforce.

"Sincerely speaking, [the fairing was] not a big difference, but I like. And is also very beautiful.

"I tried also the new tyres, something a little bit better, something a little bit worse, but not a big difference."

It could be said that Rossi was there or thereabouts through the entire test. Eighth fastest on day one, fourth on day two then fifth on the final day, all of which suggests the Italian could be on the cusp of yet another title fight.