Valentino Rossi came through practice unscathed as Dani Pedrosa set the pace at the Aragon Grand Prix on Friday.
Rossi suffered a double fracture of his leg only three weeks ago, but the Italian was declared fit to make a remarkable return at Motorland and clocked up over 20 laps.
The seven-time MotoGP champion, who only missed the Misano round following a motocross accident at the end of last month, managed eight laps in a wet FP1 which ended with Marc Marquez quickest.
Rossi, who has been spotted hobbling around the paddock with the aid of a crutch this week, was 18th on the timesheets in the first session and 20th in FP2 after completing another 13 circuits on his Yamaha – only just avoiding the gravel when he misjudged turn 12.
Pedrosa led the way after he was the only rider to dip below two minutes, the Repsol Honda man clocking 1:59.858 on a track which was still wet in certain areas.
Jorge Lorenzo (2:00.072) was second fastest in the afternoon session ahead of Johann Zarco and championship leader Marquez, with Karel Abraham fifth.
Italian duo Michele Pirro and Andrea Dovizioso were second and third respectively in FP1.
Conditions are expected to improve when the riders return to the track on Saturday.