After losing his home Grand Prix of Catalunya by celebrating one lap too early, and sinking to fourth as a result, world championship leader Julian Simon bounced back in perfect style by snatching P1 at the very end of the one hour free practice session at Assen.
The Spaniard was 0.200sec faster than Aspar Aprilia team-mate Sergio Gadea with Nicolas Terol third for Jack & Jones Aprilia.
Bradley Smith, presently just 1.5 points behind Simon in the championship, took fourth, with fellow Brit Danny Webb, riding for the home Degraaf Grand Prix team, also starting the weekend strongly in fifth position.
Catalunya winner Andrea Iannone was only ninth quickest, over one second behind Simon.
The Haojue team is once again absent.
Qualifying takes place on Friday with the race on Saturday.
Thursday practice - Assen:
1. Simon (Aprilia) 1min 46.725sec
2. Gadea (Aprilia) +0.200sec
3. Terol (Aprilia) +0.318sec
4. Smith (Aprilia) +0.388sec
5. Webb (Aprilia) +0.552sec
6. Marquez (KTM) +0.847sec
7. Vazquez (Derbi) +0.899sec
8. Cortese (Derbi) +0.994sec
9. Iannone (Aprilia) +1.049sec
10. Krummenacher (Aprilia) +1.136sec
20. Redding (Aprilia) +2.455sec