Valentino Rossi finished fifth fastest on day one at Motegi in Japan, declaring himself "happy" with the set-up of his Desmosedici.

Rossi's best time of 1m 46.687s left him less than 0.6s off the fastest Friday lap set by MotoGP title contender Dani Pedrosa on the Repsol Honda.

Factory Ducati rider Rossi said a set-up change for FP2 enabled him to get on the gas earlier on the exit of corners without spinning up and was content with his progress on opening practice day at the Twin Ring circuit.

"Today went pretty well, especially in the afternoon because we made a set-up change that helped us to improve grip on acceleration," he said.

"When I open the gas I'm able to exit the turns better, without spinning too much, and that's why I went pretty fast.

"I'm especially happy because the gap isn't too big. Tomorrow, though, we'll have to improve our pace as well, because for now I'm able to ride well for some laps, and then the tyre starts to slide a bit too much."

Rossi expects rear tyre selection on Sunday to prove crucial and the Italian will continue to assess the best option for a full race distance tomorrow.

"The rear-tyre choice will be very important because the performance of the soft and the hard are very similar," he said.

"The former works a little better but we have to understand if it can last the race distance.

"Tomorrow we'll try some more changes and then we'll also consider the temperature for Sunday when making a decision."