Czech ace Jakub Smrz comfortably topped the time-sheets during the first free practice session held in wet conditions at Miller Motorsports Park.
The Liberty Ducati rider was almost three seconds ahead of factory BMW star Marco Melandri as the sixth round of the World Superbike Championship got under way in unfavourable conditions in the United States.
Smrz left it late to seal the top spot as he lapped in a time of 2m 02.138s.
Melandri's time of 2m 04.832s kept him ahead of Sylvain Guintoli on the second Liberty Ducati, who was only a few tenths of a second adrift in third.
In a dramatic end to FP1, Honda Racing's Jonathan Rea briefly hit the front, with reigning champion Carlos Checa – who boasts an admirable record at the American venue – also edging to the top of the times before a late flurry saw Smrz, Melandri and Guintoli claim the first three places.
Ayrton Badovini was fourth fastest (2m 05.298s) from Spaniard Checa (2m 05.326s), with Nicolo Canepa rounding out the top six on his Ducati.
Checa's team-mate Davide Giugliano, factory Kawasaki rider Tom Sykes – who had been lying in second earlier in the session – and Northern Irish duo Rea and Eugene Laverty completed the top ten, four seconds in arrears.
Laverty's works Aprilia team-mate and championship leader Max Biaggi struggled with his RSV-4 at Miller and ended FP1 in a lowly 20th position.
Briton Leon Haslam was 12th on his BMW S1000RR ahead of Chaz Davies, while American John Hopkins was 17th on the Fixi Crescent Suzuki, finishing ahead of team-mate Leon Camier in 22nd.
to view full FP1 times