Championship leader Marc Marquez posted the quickest time in Friday's combined free practice ahead of Sunday's Aragon MotoGP.
The 20-year-old MotoGP rookie took the free practice honours on his Honda, clocking a best lap of 1min 48.987sec.
Next best was Germany's Stefan Bradl and Marquez's teammate Dani Pedrosa, the winner last season, in third.
Yamaha's defending champion Jorge Lorenzo, quickest in the morning's first session, came in fourth.
"Today, I didn't feel as good as I did in the test that we did here after the Barcelona GP, as there was less grip in these two practice sessions," Marquez said.
He added: "Anyway, it's the same for everyone. In general the feeling that we have is pretty good and I worked on various things with the bike today.
"However, we can still improve our setup. We hope to take that step forward tomorrow and we shall see if the grip level improves, as that would make things so much easier for everyone."
Lorenzo, and his teammate, seventh-placed Valentino Rossi, know they have work to do to challenge their Honda rivals.
"We hoped to improve the bike from the morning but we couldn't do it," said Lorenzo.
"I have a worse feeling than when we were here for the test. We have to keep working to improve the bike and the pace."
With five wins under his belt Marquez has a 34 point lead in the world championship standings over Lorenzo and Pedrosa.