Spain's Dani Pedrosa finished top after day one of practice at the Japanese MotoGP ahead of World Championship leader Casey Stoner in Motegi.
Stoner is on the brink of becoming double world champion, holding a 44-point lead over Jorge Lorenzo.
Andrea Dovizioso was third quickest as Repsol Honda took the top three spots.
The meeting was originally scheduled for April but was postponed as a result of the earthquake and tsunami which hit the region back in March.
Several riders had pondered boycotting the race because of fears for their health due to radiation levels in Motegi, which is just over 120 kilometres from Fukushima.
Australian Stoner was quickest in first practice, setting the fastest lap of the session with one minute 47.289 seconds ahead of team-mate Dovizioso.
Pedrosa took over top spot with 21 minutes to go of the second session and set a new fast lap of one minute 46.790 secs to finish ahead of team-mate Stoner by a margin of 0.054s.
Pedrosa said: "We did a good job. In the morning we took a good direction and this allowed us to improve a lot in the afternoon, by a second, and this is very important."