Third fastest while using a 2019-spec Yamaha engine on the opening day of the Valencia MotoGP test, Valentino Rossi slipped down to ninth (+0.6s) on the final day while trying a second new powerplant.

"We were able to improve compared to yesterday, but unfortunately our opponents were able to improve more," Rossi said.

"With the new tyre we are not so bad, but after some laps we suffer more and our pace is not fantastic."

Rossi said the second new engine offered little difference compared to the first version used on Tuesday, as Yamaha focuses on smoother acceleration and braking to extend tyre life.

"The main issue was to try the second [2019] engine spec. We tried back-to-back, but sincerely there was not a big difference so we have more or less the same performance," he said.

"Yesterday there was more difference, with the [first new] engine compared to [the 2018 engine]. But the engine of today is very similar to the engine of yesterday.

"We'll continue with this [engine] material in Jerez, to try to understand and decide which one is the best and give some indication to Yamaha.

"Jerez is an important test because in the grand prix in May we were not very fast, so we need to understand if we are strong."

Team-mate Maverick Vinales, who set the fastest time on both days, also said it was "difficult" to decide between the two engines.

Rossi is unsure if a third version might become available early next year, or if the 2019 race engine is a straight choice between the versions currently on offer.

"I don't know," shrugged Rossi. "We have time also for another. But I don’t know the plan of Yamaha, if it's this or in February they can bring something better…"

The Jerez test takes place from November 28-29, with the winter test ban starting on December 1.