World championship leader Alvaro Bautista got his Assen weekend off to a flying start by leading Thursday free practice for the 250cc Dutch TT.
The Aspar Aprilia rider, who will start round seven 12 points ahead of Hiroshi Aoyama, held off countryman Hector Barbera by 0.185sec with rookie Mike di Meglio a strong third. Aoyama was only eighth fastest.
Reigning 250cc world champion Marco Simoncelli, confirmed as a 2010 MotoGP rider with Gresini Honda on Wednesday, crashed while leading with 23 minutes to go. The Italian was unharmed but forced to abandon his number one bike and slipped down to seventh in the closing stages.
Mugello winner Mattia Pasini made a late start to the session but recovered to claim fifth in practice.
Qualifying takes place on Friday with the race on Saturday.
Thursday practice - Assen:
1. Bautista (Aprilia) 1min 40.817sec
2. Barbera (Aprilia) +0.185sec
3. di Meglio (Aprilia) +0.655sec
4. Debon (Aprilia) +0.863sec
5. Pasini (Aprilia) +0.893sec
6. Pesek (Aprilia) +1.015sec
7. Simoncelli (Gilera) +1.049sec
8. Aoyama (Honda) +1.124sec
9. Espargaro (Aprilia) +1.665sec
10. Luthi (Aprilia) +1.685sec
24. Markham (Honda) +6.429sec