Cal Crutchlow beat Valentino Rossi to the top spot in opening MotoGP practice at a cool and misty Laguna Seca.
Rossi had led the way going into the closing minutes, but a late 1m22.757s from Crutchlow put the Tech 3 Yamaha ahead of the works version by just 0.091 seconds.
Championship leader Marc Marquez worked his way up to third place as he learned the Laguna track on his first visit. The Honda rider was only 0.223s off the pace.
Having received medical clearance to take to the track just before the session, Jorge Lorenzo did eight cautious laps as he assessed his injured shoulder.
He was classified 11th, 1.6s off Crutchlow.
Fellow injured title contender Dani Pedrosa was also cleared to ride, but chose to sit out first practice to avoid straining himself. He will take to the track in the afternoon.
Stefan Bradl had a spell on top early on and was Rossi's closest rival for a long while, before slipping to fourth on the LCR Honda.
Ducati's Andrea Dovizioso also spent a while in first place and completed the final top six, behind Alvaro Bautista.
Dovizioso's team-mate Nicky Hayden, who announced on Thursday that he is losing his ride at the end of the year, was the leading American in eighth.
That put him behind top CRT rider Aleix Espargaro.
But it was not a straightforward session for Espargaro. He slid over the sand trap at the Corkscrew on one lap and had to park his Aprilia trackside at the end of the session.
His team-mate Randy de Puniet, who ended the morning 10th behind Bradley Smith, incurred the wrath of Crutchlow when they had a near-miss mid-session.
Alex de Angelis began his MotoGP return in 13th place. The Moto2 frontrunner is standing in for Ben Spies on what has become the sole Pramac Ducati due to Andrea Iannone's Sachsenring injury.
Local wildcard Blake Young was back with the Attack team's CRT bike and took 21st place.
Pos Rider Team/Bike Time Gap
1. Cal Crutchlow Tech 3 Yamaha 1m22.757s
2. Valentino Rossi Yamaha 1m22.848s +0.091s
3. Marc Marquez Honda 1m22.980s +0.223s
4. Stefan Bradl LCR Honda 1m23.033s +0.276s
5. Alvaro Bautista Gresini Honda 1m23.255s +0.498s
6. Andrea Dovizioso Ducati 1m23.380s +0.623s
7. Aleix Espargaro Aspar Aprilia 1m23.905s +1.148s
8. Nicky Hayden Ducati 1m23.927s +1.170s
9. Bradley Smith Tech 3 Yamaha 1m24.287s +1.530s
10. Randy de Puniet Aspar Aprilia 1m24.363s +1.606s
11. Jorge Lorenzo Yamaha 1m24.441s +1.684s
12. Hector Barbera Avintia FTR-Kawasaki 1m24.467s +1.710s
13. Alex de Angelis Pramac Ducati 1m25.133s +2.376s
14. Yonny Hernandez PBM Aprilia 1m25.309s +2.552s
15. Karel Abraham Cardion Aprilia 1m25.394s +2.637s
16. Hiroshi Aoyama Avintia FTR-Kawasaki 1m25.451s +2.694s
17. Danilo Petrucci Ioda-Suter-BMW 1m25.594s +2.837s
18. Colin Edwards Forward FTR-Kawasaki 1m25.664s +2.907s
19. Claudio Corti Forward FTR-Kawasaki 1m25.770s +3.013s
20. Michael Laverty PBM-Aprilia 1m26.659s +3.902s
21. Blake Young Attack APR-Kawasaki 1m26.668s +3.911s
22. Bryan Staring Gresini FTR-Honda 1m26.753s +3.996s
23. Lukas Pesek Ioda-Suter-BMW 1m27.825s +5.068s
24. Dani Pedrosa Honda