125cc World Championship leader Nico Terol overturned Maverick Vinales' morning advantage to finish Friday practice on top of the timesheets at Aragon, Spain.
Aspar Aprilia star Terol, who had been 0.266sec behind Vinales this morning, concluded the afternoon session with a 0.365sec advantage over the rookie.
Fellow 2011 race winners' Sandro Cortese and Hector Faubel were third and fourth respectively, followed by Ajo Derbi team-mates Efren Vazquez and Johann Zarco.
Zarco starts this weekend 31 points behind Terol, having once again been narrowly denied a debut race victory last time out at Misano.
Vinales' former Paris Hilton Aprilia team-mate Sergio Gadea has made a surprise switch to Moto2 (on a G22 Moriwaki) this weekend and been replaced by Spanish youngster Josep Rodriguez.
Rodriguez, making his fourth grand prix appearance, was 20th fastest on Friday afternoon.
This afternoon's session was delayed by a power cut. A second power cut has since put the MotoGP and Moto2 FP2 sessions in doubt.
Friday practice (2):
1. Terol (Aprilia) 1m 59.409s
2. Vinales (Aprilia) +0.365s
3. Cortese (Aprilia) +0.756s
4. Faubel (Aprilia) +1.301s
5. Vazquez (Derbi) +1.342s
6. Zarco (Derbi) +1.356s
7. Kent (Aprilia) +1.744s
8. Oliveira (Aprilia) +1.994s
9. Martin (Aprilia) +2.045s
10. Salom (Aprilia) +2.140s
16. Stafford (Aprilia) +2.799s
18. Webb (Mahindra) +3.198s
29. Mackenzie (Aprilia) +4.989s