Championship leader Casey Stoner registered the fastest time in practice for Sunday's United States MotoGP in Indianapolis.
The Australian posted one minute 40.724 seconds in second practice, 0.194sec better than American Ben Spies, who had led the first session.
Last year's race winner Dani Pedrosa finished the session in third.
Stoner's nearest rival in the championship standings, Spaniard Jorge Lorenzo, was fourth.
Yamaha rider Lorenzo's time in the second session was a significant improvement on his first practice, bettering his time by over three seconds.
Stoner's second practice time was also a huge improvement on his first, allowing him to see off what was a solid challenge from Yamaha's Spies, who finished on pole here last year before claiming second in the race.
Stoner said: "I'm still not comfortable on this surface. It's incredibly slippery and the track is certainly my least favourite of the whole season.
"It's very tight, slippery and the new surface is pretty bad, it's not just the dirt on the surface but there seems to be something strange where the two surfaces join, some grease of some sort that creates more issues.
"My last few exits were slightly ruined from traffic on track and also wildlife! I saw something out the corner of my eye, I'm not sure what it was perhaps a gopher or something, I thought it was a part from someone's bike, I'm just glad we both escaped from that situation unharmed!"
Italian Andrea Dovizioso, who is third in the championship race, 55 points behind Stoner, was fifth fastest on his Honda following a three second improvement from first practice, leaving him 0.163sec ahead of American Yamaha rider Colin Edwards.
Former world champion Valentino Rossi was down in 11th. He won the inaugral event at Indianapolis in 2008 but has yet to win a race since switching from Yamaha to Ducati this season.
Britain's Cal Crutchlow came in 15th.
US MotoGP first free practice:
1. Ben Spies (US/Yamaha) 1min 43.260sec
2. Casey Stoner (Aus/Honda) at 0.292sec
3. Colin Edwards (US/Yamaha) 0.314
4. Marco Simoncelli (Ita/Honda) 0.322
5. Nicky Hayden (US/Ducati) 0.976
6. Valentino Rossi (Ita/Ducati) 1.102
7. Andrea Dovizioso (Ita/Honda) 1.291
8. Hector Barbera (Spa/Ducati) 1.766
9. Jorge Lorenzo (Spa/Yamaha) 1.805
10. Dani Pedrosa (Spa/Honda) 1.932
11. Alvaro Bautista (Spa/Suzuki) 1.989
12. Cal Crutchlow (GB/Yamaha) 2.108
13. Randy de Puniet (Fra/Ducati) 2.317
14. Hiroshi Aoyama (Jpn/Honda) 2.899
15. Loris Capirossi (Ita/Ducati) 3.224
16. Karel Abraham (Cze/Ducati) 3.383
17. Toni Elias (Spa/Honda) 4.247
Second free practice:
1. Stoner 1min 40.724sec
2. Spies at 0.194sec
3. Pedrosa 0.481
4. Lorenzo 0.737
5. Dovizioso 0.812
6. Edwards 0.975
7. Simoncelli 1.018
8. Hayden 1.065
9. Bautista 1.507
10. De Puniet 1.615
11. Rossi 1.681
12. Aoyama 1.949
13. Barbera 2.196
14. Capirossi 2.310
15. Crutchlow 2.361
16. Elias 2.506
17. Abraham 2.804