Dani Pedrosa, winner of the last two races, finished on top of the combined Aragon practice timesheets after the first day of MotoGP action on the new Spanish circuit.
The newly re-signed Repsol Honda rider led the way courtesy of his pace-setting morning time, with the afternoon session - re-introduced as a trial for this event - being held in wet conditions.
Pedrosa led the first half of the wet 45-minute outing, but had dropped to 13th by the flag after an electronic problem prevented him being on track when conditions improved.
"In general this has been a good first day of practice for us at a brand new circuit for MotoGP," said Pedrosa, who is 63 points behind Jorge Lorenzo in the championship.
"This morning we started well and got into a rhythm quickly, which was important because it makes a big difference going from the one hour practice sessions we're familiar with to 45 minutes. It's necessary to stay out on track for longer runs and not spend too long in the garage, and this morning we did a good job to maximise our time.
"This afternoon in the wet we also started pretty well when it was raining properly, but then we had a small electrical problem with one bike and the other machine wasn't ready to go out, so we lost some time in the garage and I wasn't on track when the surface was improving at the end.
"The track is quite technical; it has some difficult turns and you really have to find a good line and also to set up the gearbox properly to be able to make a good lap time. The grip from the track surface feels good and the Bridgestone tyres are also working well so far. I hope we can continue in this positive way tomorrow.”
Pedrosa has lost out to countryman Lorenzo at the two previous Spanish rounds this year.
Pedrosa's team-mate Andrea Dovizioso was eighth fastest.
"Overall the work we did today was positive, although I'm not totally happy with the final lap times we achieved. We started well enough this morning but then we weren't able to get the maximum from our package, which means we can improve tomorrow," he said.