Marc Marquez continued to set the pace at Laguna Seca after going quickest in the third practice session for the United States Grand Prix on Saturday.
The Honda rider, on his first visit to the Californian track, posted a best time of 1m21.568s to finish ahead of Yamaha rival Valentino Rossi, who finished second thanks ro a late improvement.
LCR's Stefan Bradl was third quickest and the only other rider to lap in the 1m21s.
Alvaro Bautista and Cal Crutchlow completed the top five ahead of Ducati's Andrea Dovizioso and world champion Jorge Lorenzo, who completed just 11 laps on his Yamaha.
Championship contender Dani Pedrosa, who had opted not to run in the first practice session, decided against going out once more to avoid straining himself. The Spanish Honda rider will need to go through Q1 after setting only the 11th fastest time in practice two.
Local hero Nicky Hayden was the early pacesetter during a cold start to the session, the Ducati rider soon outpaced by Rossi before Crutchlow jumped to the top of the times.
The Briton soon posted the fastest time of the weekend so far with a lap of 1m22.314s, but Marquez took charge minutes later, becoming the first man to lap in the 1m21s despite a scare under braking for the Corkscrew.
Lorenzo, still recovering from his injury, did not take to the track until there were around 25 minutes of the session left. He completed a five-lap run that allowed him to jump to third position.
With 20 minutes left, Marquez went even quicker, setting a new benchmark of 1m21.568s, over six tenths of a seconds quicker than his closest rival at that point.
Gresini rider Bautista moved to second place with less than 10 minutes to go, with Bradl also improving to jump to third. Marquez was finally joined in the 1m21s by Bradl, who went second quickest in the final minutes with a lap of 1m21.959s.
Rossi also went off track near the end of the session, but the Italian rejoined without further drama and moved into second place with less than a minute to go.
Aleix Espargaro was once again the quickest CRT rider, finishing ninth overall.
Pramac rider Alex de Angelis was the only man to be involved in an accident after he went off track before the Corkscrew, falling as he tried to return to the track. He managed to get back to the pits with the damaged bike.
Pos Rider Team/Bike Time Gap 1. Marc Marquez Honda 1m21.568s 2. Valentino Rossi Yamaha 1m21.752s +0.184s 3. Stefan Bradl LCR Honda 1m21.959s +0.391s 4. Alvaro Bautista Gresini Honda 1m22.085s +0.517s 5. Cal Crutchlow Tech 3 Yamaha 1m22.187s +0.619s 6. Andrea Dovizioso Ducati 1m22.226s +0.658s 7. Jorge Lorenzo Yamaha 1m22.290s +0.722s 8. Nicky Hayden Ducati 1m22.504s +0.936s 9. Aleix Espargaro Aspar Aprilia 1m22.733s +1.165s 10. Bradley Smith Tech 3 Yamaha 1m22.812s +1.244s 11. Randy de Puniet Aspar Aprilia 1m23.269s +1.701s 12. Alex de Angelis Pramac Ducati 1m23.616s +2.048s 13. Danilo Petrucci Ioda-Suter-BMW 1m24.057s +2.489s 14. Colin Edwards Forward FTR-Kawasaki 1m24.108s +2.540s 15. Hiroshi Aoyama Avintia FTR-Kawasaki 1m24.460s +2.892s 16. Hector Barbera Avintia FTR-Kawasaki 1m24.618s +3.050s 17. Yonny Hernandez PBM Aprilia 1m24.670s +3.102s 18. Karel Abraham Cardion Aprilia 1m24.761s +3.193s 19. Claudio Corti Forward FTR-Kawasaki 1m24.895s +3.327s 20. Michael Laverty PBM-Aprilia 1m24.946s +3.378s 21. Bryan Staring Gresini FTR-Honda 1m25.220s +3.652s 22. Lukas Pesek Ioda-Suter-BMW 1m26.979s +5.411s 23. Dani Pedrosa Honda