Valentino Rossi says he has plenty of work to do ahead of the Japanese MotoGP on Sunday after an unimpressive start to the weekend in practice.

The championship leader finished eighth in second practice on Friday as he struggled to find the right setting on his Yamaha bike, while his team-mate and title rival Jorge Lorenzo went fastest.

Lorenzo was quickest in both sessions in Motegi, with Rossi's switch to softer tyres failing to have the desired impact on his lap times.

"This morning was not so bad and also in the afternoon I felt quite good with the harder tyre," said Rossi – who finished third in P1. 

"I expected to improve more when I put on the softer option at the end, but unfortunately this didn't happen because we're not good with the setting yet and have to work a bit further." 

Meanwhile, Lorenzo was surprised his injury problems did not put him at a disadvantage but knows he will have to keep pushing to stay ahead of Rossi.

He said: "I didn't expect to be so competitive. I expected to improve little by little, but starting from the first run I was the fastest rider and I improved even more on the last outing.

"The bike is going better than last year and we have a great setup. Obviously my shoulder isn't perfect, but even with my shoulder in this condition I'm still able to ride and be quite consistent.

"I believe that with normal physical conditions I could have been even faster, but there's no point in thinking about what could have happened. It's better to focus on the current situation and take the maximum profit.

"For tomorrow [Saturday] we still have a margin to improve the bike to suit my riding."