Marc Marquez left his MotoGP rivals trailing in the opening Friday practice session at Austin.
The world champion's Honda was over a second clear of the field until improvements from Aleix Espargaro and Dani Pedrosa reduced Marquez's advantage to a relatively small 0.887 seconds.
Marquez's opening runs had left him 1.6s faster than anyone else early on. He eventually got down to a 2m04.704s at the track where he took his maiden MotoGP win in only his second start 12 months ago.
Until his final run, Open class rider Espargaro had been outside the top 10 and looking unlikely to repeat his giantkilling practice form from Qatar last month.
But his late jump up the order allowed the Forward Racing man to split the factory Hondas in the top three.
Espargaro and Pedrosa's progress relegated Valentino Rossi to fourth on the top Yamaha.
Rossi's team-mate Jorge Lorenzo was back in 12th and again appeared to be expressing his unhappiness with the bike's handling in the pits between runs.
Andrea Dovizioso completed the top five on the fastest Ducati.
His team-mate Cal Crutchlow claimed seventh in the end behind Bradley Smith, despite having to abandon an early run and 'scoot' his bike back down the pitlane after a technical problem emerging as he tried to set off.
Gresini's Alvaro Bautista and Aspar's Nicky Hayden also had technical issues during the morning, Bautista's coming right at the start of his out-lap.
Pos Rider Bike Time Gap Laps
1. Marc Marquez Honda 2m04.704s 15
2. Aleix Espargaro Forward Yamaha 2m05.591s +0.887s 13
3. Dani Pedrosa Honda 2m05.676s +0.972s 15
4. Valentino Rossi Yamaha 2m05.972s +1.268s 15
5. Andrea Dovizioso Ducati 2m06.279s +1.575s 14
6. Bradley Smith Tech 3 Yamaha 2m06.336s +1.632s 18
7. Cal Crutchlow Ducati 2m06.433s +1.729s 15
8. Stefan Bradl LCR Honda 2m06.537s +1.833s 15
9. Andrea Iannone Pramac Ducati 2m06.602s +1.898s 14
10. Alvaro Bautista Gresini Honda 2m06.633s +1.929s 13
11. Pol Espargaro Tech 3 Yamaha 2m06.680s +1.976s 17
12. Jorge Lorenzo Yamaha 2m06.771s +2.067s 14
13. Nicky Hayden Aspar Honda 2m07.450s +2.746s 13
14. Colin Edwards Forward Yamaha 2m07.644s +2.940s 15
15. Yonny Hernandez Pramac Ducati 2m07.689s +2.985s 16
16. Karel Abraham Cardion Honda 2m07.702s +2.998s 13
17. Hector Barbera Avintia FTR-Kawasaki 2m08.150s +3.446s 16
18. Hiroshi Aoyama Aspar Honda 2m08.455s +3.751s 17
19. Scott Redding Gresini Honda 2m08.588s +3.884s 14
20. Danilo Petrucci Ioda Aprilia 2m09.646s +4.942s 13
21. Broc Parkes PBM Aprilia 2m10.206s +5.502s 15
22. Michael Laverty PBM Aprilia 2m10.493s +5.789s 12
23. Mike di Meglio Avintia FTR-Kawasaki 2m11.100s +6.396s 15