Marc Marquez immediately showed he felt at home on Friday as he set the fastest time in the first practice on the Aragon track where he has won for the last three years.
The Honda rider lead Spanish compatriot Maverick Vinales (Yamaha) by 1.6 seconds.
Marquez, who is on course for a sixth MotoGP world title, did not make any errors as he lapped the 5.077km circuit in 1 min 46.869sec, just 0.34sec outside the lap record he set in 2015.
Frenchman Fabio Quartararo, on the SRT satellite team's Yamaha, set the third fastest time ahead of Andrea Dovizioso (Ducati) and Australian Jack Miller (Ducati-Pramac).
Finland's Mika Kallio, who replaced Frenchman Johann Zarco at KTM for the rest of the season, was 19th fastest.
1. Marc Marquez (ESP/Honda): 1min 46.869sec, 2. Maverick Vinales (ESP/Yamaha) at 1.617, 3. Fabio Quartararo (FRA/Yamaha-SRT) 2.001, 4. Andrea Dovizioso (ITA/Ducati) 2.020, 5. Jack Miller (AUS/Ducati-Pramac) 2.232, 6. Joan Mir (ESP/Suzuki) 2.232, 7. Danilo Petrucci (ITA/Ducati) 2.361, 8. Pol Espargaro (ESP/KTM) 2.431, 9. Franco Morbidelli (ITA/Yamaha-SRT) 2.435, 10. Takaaki Nagakami (JPN/Honda-LCR) 2.574