Repsol Honda rider Dani Pedrosa was quickest in free practice for the Indianapolis Grand Prix, with team-mate Casey Stoner fifth fastest.
Stoner led the field in the opening session before Spaniard Pedrosa set the fastest time of the day, recording a one minute 39.783 seconds-lap.
Ben Spies registered the second-quickest time in his home country, 0.295 seconds behind Pedrosa with his Yamaha Factory Racing team-mate, and championship leader, Jorge Lorenzo third fastest.
Tech 3 Yamaha's Andrea Dovizioso was fourth, while British rider Cal Crutchlow recovered from a crash in the morning session, that saw him take to his second bike, to post the sixth-best time, 1.272 secs behind Pedrosa.
Pramac Ducati rider Hector Barbera was taken to hospital with fractured vertebrae following a crash and is ruled out for Sunday's race. He will be replaced by Toni Elias.
Spaniard Barbera had only just returned to the track after breaking both bones in his lower left leg in July.
Second practice times and standings for Indianapolis Grand Prix.
1 Dani Pedrosa (Spa) Repsol Honda One minute 39.783 seconds
2 Ben Spies (US) Yamaha 1:40.078
3 Jorge Lorenzo (Spa) Yamaha 1:40.502
4 Andrea Dovizioso (Ita) Tech 3 Yamaha 1:40.560
5 Casey Stoner (Aus) Repsol Honda 1:40.623
6 Cal Crutchlow (GB) Tech 3 Yamaha 1:41.055
7 Alvaro Bautista (Spa) Honda Gresini 1:41.072
8 Stefan Bradl (Ger) LCR Honda 1:41.255
9 Nicky Hayden (US) Ducati 1:41.397
10 Valentino Rossi (Ita) Ducati 1:41.529