Pol Espargaro beat his older brother Aleix to take the early advantage as MotoGP's Assen weekend kicked off with free practice on Thursday morning.
The Tech 3 rookie had remained anonymous for much of the 45-minute session before embarking upon his final run, when he began to rise up the order with a succession of quick laps.
Espargaro's fastest came moments before the chequered flag and his time of 1m34.530s was good enough to dispose of his sibling from the top of the timesheets by 0.189s. Aleix set his best while running on the soft tyre available to beat his Open Class competitors.
Jorge Lorenzo was quickest of the factory riders and finished ahead of Marc Marquez, who led for much of the session after spending 15 minutes in the garage as the Honda mechanics attended to the front of his bike in secrecy following an off-track excursion.
Bradley Smith enjoyed a brief spell at the top of the order, but the Briton made no further improvements after posting his best halfway through the session and ended the morning fifth, with last year's race winner Valentino Rossi in sixth.
Andrea Dovizioso and Dani Pedrosa traded personal quickest times as the clocked ticked down, but it was the Italian who triumphed to claim seventh on his Ducati.
Team-mate Cal Crutchlow's difficult season continues as he finished the session in 13th and 1.765s off the ultimate pace.
Danilo Petrucci returned to action after recovering from his fractured left wrist he sustained at Jerez last month.
The Ioda Racing rider propped up the order with a time of 1m37.732s.
Pos Rider Bike Time Gap Laps
1. Pol Espargaro Tech 3 Yamaha 1m34.530s 21
2. Aleix Espargaro Forward Yamaha 1m34.719s +0.189s 17
3. Jorge Lorenzo Yamaha 1m34.736s +0.206s 19
4. Marc Marquez Honda 1m34.770s +0.240s 18
5. Bradley Smith Tech 3 Yamaha 1m34.915s +0.385s 21
6. Valentino Rossi Yamaha 1m34.974s +0.444s 21
7. Andrea Dovizioso Ducati 1m35.069s +0.539s 18
8. Dani Pedrosa Honda 1m35.187s +0.657s 18
9. Alvaro Bautista Gresini Honda 1m35.364s +0.834s 19
10. Stefan Bradl LCR Honda 1m35.885s +1.355s 21
11. Andrea Iannone Pramac Ducati 1m35.888s +1.358s 19
12. Colin Edwards Forward Yamaha 1m36.269s +1.739s 19
13. Cal Crutchlow Ducati 1m36.295s +1.765s 20
14. Scott Redding Gresini Honda 1m36.353s +1.823s 18
15. Yonny Hernandez Pramac Ducati 1m36.363s +1.833s 19
16. Karel Abraham Cardion Honda 1m36.375s +1.845s 16
17. Hiroshi Aoyama Aspar Honda 1m36.521s +1.991s 20
18. Broc Parkes PBM Aprilia 1m37.209s +2.679s 14
19. Nicky Hayden Aspar Honda 1m37.275s +2.745s 18
20. Hector Barbera Avintia FTR-Kawasaki 1m37.370s +2.840s 17
21. Michael Laverty PBM Aprilia 1m37.488s +2.958s 18
22. Mike di Meglio Avintia FTR-Kawasaki 1m37.498s +2.968s 20
23. Danilo Petrucci Ioda Suter-BMW 1m37.732s +3.202s 16