Rossi is launching a determined late bid to hold on to his crown

Rossi, bidding to overtake championship leader Nicky Hayden and win his sixth straight title, topped the timesheets from the day's two practice sessions.

The Italian clocked one minute 46.981 seconds in the afternoon, with Repsol Honda rider Hayden trailing in fifth.

Japan's Kawasaki rider Shinya Nakano was second fastest, 0.334 seconds back, with Italian Marco Melandri third.

With three rounds of the season to go, former US Superbikes champion Hayden leads Rossi by 21 points.

The starting grid for the season's 15th round will be decided by the official qualifying session following the third free practice on Saturday.

Rossi, 27, has been hit by a run of spills and tyre and engine troubles this season but still claimed five wins.

Hayden, 25, has won two races this season, in Holland and the USA, with seven other podium finishes, and has scored points in every race so far.

Japanese MotoGP leading practice times:
1. Valentino Rossi (It) Yamaha 1min 46.981sec
2. Shinya Nakano (Jpn) Kawasaki 1:47.315
3. Marco Melandri (It) Honda 1:47.449 4.
4. Colin Edwards (USA) Yamaha, 1:47.581.
5. Nicky Hayden (USA) Honda, 1:47.610.
6. Toni Elias (Sp) Honda 1:47.646.
7. Loris Capirossi (It) Ducati 1:47.731.
8. John Hopkins (USA) Suzuki 1:47.915.
9. Randy De Puniet (Fr) Kawasaki 1:47.982.
10 Kenny Roberts Jr. (USA) KR211V 1:48.002.