Nico Rosberg topped the timesheet in FP1 in Austin in a session where the drivers were hampered by a limited number of inters and a slowly drying track.
It was a wet start to the weekend's action at the Circuit of the Americas but by no means the wettest that it is forecast to be.
Although the track was soaked ahead of FP1 without any fresh rain falling and no standing water the drivers were almost immediately onto the intermediate tyres.
Rosberg set the first competitive time of a 1:55.102 while the Red Bulls of Daniil Kvyat and Daniel Ricciardo lined up behind him.
The Mercedes driver, still on the inters, returned to the track to clock a 1:53.989 to extend his advantage to 1.2s over Kvyat while Ricciardo was a further 0.4s down.
With the track too dry for inters, of which the drivers have only a limited number, and too wet for slicks the action slowed down as the session continued.
The conditions did not favour much running in the final 30 minutes and Rosberg ended the session P1 ahead of the Red Bulls. He covered just seven laps in the 90 minutes.
Carlos Sainz, fit and ready after his horrific Sochi crash, was fourth quickest ahead of Lewis Hamilton and Sebastian Vettel. The Mercedes driver and the Ferrari man were both 1.7s down on Rosberg's best.
The session concluded without a time on the board from Pastor Maldonado. Returning to the pits after his installation lap his E23 was soon stripped of its floor and later its gearbox.
1 Nico Rosberg Mercedes 1:53.989 7 laps
2 Daniil Kvyat Red Bull 1:55.224 +1.235 6 laps
3 Daniel Ricciardo Red Bull 1:55.592 1.603 10 laps
4 Carlos Sainz Toro Rosso 1:55.667 1.678 8 laps
5 Lewis Hamilton Mercedes 1:55.693 1.704 4 laps
6 Sebastian Vettel Ferrari 1:55.710 1.721 10 laps
7 Max Verstappen Toro Rosso 1:55.969 1.980 7 laps
8 Kimi Raikkonen Ferrari 1:56.326 2.337 10 laps
9 Fernando Alonso McLaren 1:56.839 2.850 18 laps
10 Valtteri Bottas Williams 1:56.866 2.877 15 laps
11 Felipe Massa Williams 1:57.081 3.092 13 laps
12 Sergio Perez Force India 1:57.139 3.150 4 laps
13 Jenson Button McLaren 1:57.495 3.506 20 laps
14 Nico Hulkenberg Force India 1:57.518 3.529 4 laps
15 Romain Grosjean Lotus 1:58.319 4.330 10 laps
16 Raffaele Marciello Sauber 1:59.431 5.442 19 laps
17 Marcus Ericsson Sauber 1:59.743 5.754 18 laps
18 Alexander Rossi Manor 2:01.154 7.165 11 laps
19 Will Stevens Manor 2:01.907 7.918 13 laps
20 Pastor Maldonado Lotus No time 1 lap