Kawasaki's Rea has already won the first four rounds of the new campaign, and posted a one minute 50.062 lap late in Friday's second free practice session to impose his authority in Aragon.
Marco Melandri and Tom Sykes completed the top three on the timesheets, ahead of Saturday's race one showdown.
Haslam, Lowes involved in early crash as Race 2 red-flagged
Jonathan Rea has opened up a 30-point lead at the top of the Superbike World Championship
Image credit: PA Sport
The riders will complete the third and final free practice on Saturday morning, before moving on to qualifying, with race one slated for a 1pm start.
Tom Sykes hospitalised after heavy crash in Catalunya, suffers concussion but nothing else
Rinaldi wins Race 2 in Catalunya as Toprak retakes championship lead