Spaniard Dani Pedrosa lapped fastest for Honda ahead of his hometown race but Valentino Rossi, returning three weeks after breaking his leg, was the centre of attention in a wet Aragon MotoGP practice.
"I'm quite happy, because I feel comfortable on the bike," said Yamaha's 38-year-old Italian, who suffered the injury while training on an off-road enduro bike.
"The leg, after two practice sessions, is in a good condition, I don't feel a lot of pain.
"For sure on the wet it's a bit easier, because it's less demanding to ride the bike, but I hope we will have better weather conditions tomorrow and on Sunday, in order to try to understand my level on the dry and also so we can work on the setting and the tyres."
Rossi made a tentative start to the opening session on rain tyres, before completing 13 laps in the second stint to end up 20th on the combined timesheets.
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Spanish teammate Maverick Vinales was 17th.
"We didn't know what to expect from Vale on this first day of riding, but the wet conditions gave him a smooth transition coming back from injury," said team director Massimo Meregalli.
"Today's goal was for him to get familiarised with the bike again. However, we are expecting dry conditions tomorrow and only then will we truly know where he stands."
Pedrosa was the only rider to lap in under two minutes, with a best effort of 1:59.858. Reigning champion and Honda teammate Marc Marquez was fourth.
Marquez and Italian Andrea Dovizioso, on a Ducati, are tied at the top of the overall standings.
