Late progress by Dani Pedrosa saw the home rider finish first practice for the Aragon MotoGP with a 0.449sec advantage over the rest of the field.
The Repsol Honda rider, second in last year's race, moved ahead of countryman and reigning world champion Jorge Lorenzo with two-minutes remaining, then improved further on his 17th and final lap.
Lorenzo, looking to continue momentum from his third victory of the season last time at Misano, kept his Yamaha in second place, just 0.076sec ahead of Pedrosa's team-mate and world championship leader Casey Stoner.
Stoner starts round 14 with a 35 point lead over Lorenzo.
Gresini Honda's Marco Simoncelli was fourth fastest this morning (+0.612sec) with third Repsol rider Andrea Dovizioso (+0.810sec) the final person within one-second of Pedrosa.
Aragon was Ducati's most successful event of 2010, with Stoner and Nicky Hayden finishing first and third. This morning, the top Desmosedici was satellite Pramac rider Randy de Puniet in sixth (+1.095sec), followed by factory riders Hayden (+1.137sec) and Valentino Rossi (+1.395sec).
Rossi has an aluminium (rather than carbon fibre) front chassis to try on one of his Desmosedicis this weekend.
Monster Yamaha Tech 3's Colin Edwards began the weekend in ninth place (1.545sec) with Hiroshi Aoyama tenth (+1.578sec) for Gresini.
The rider most out of place this morning was Lorenzo's team-mate Ben Spies, the American concluding the session in eleventh, 1.630sec behind Pedrosa.
Second practice takes place this afternoon.
Friday practice (1):
6. de Puniet