Pol Espargaro set the 125cc pace on a wet track during Friday's second practice session at the new Aragon circuit in Spain.
The Tuenti Derbi rider had been beaten by Nico Terol this morning, in a session interrupted by the arrival of bad weather, but got the better of title leader Marc Marquez by 0.644sec in the afternoon.
Marquez - who starts round 12 with a nine point lead over Terol and 20 point advantage over Espargaro - had been just 23rd in the morning after failing to get a quick time in before the weather deteriorated.
Ongetta Team's Jonas Folger used the slick conditions to claim a better than usual third place in the afternoon, with Aspar Aprilia team-mates Terol and Bradley Smith fourth and fifth.
Friday practice (2) - Aragon:
1. Espargaro (Derbi) 2m 13.224s
2. Marquez (Derbi) +0.644s
3. Folger (Aprilia) +0.944s
4. Terol (Aprilia) +1.178s
5. Smith (Aprilia) +1.281s
6. Cortese (Derbi) +1.555s
7. Webb (Aprilia) +1.725s
8. Salom (Aprilia) +1.757s
9. Martin (Aprilia) +3.058s
10. Schrotter (Honda) +3.058s