Nico Rosberg topped the opening practice session for the US Grand Prix ahead of the Red Bull Racing pairing of Daniil Kvyat and Daniel Ricciardo, but when the damp morning conditions gave way to torrential afternoon downpours, the second session at the Circuit of the Americas was cancelled. 

Bad weather had been forecast for the Austin area overnight and although the rain showers had abated by the start of the weekend’s opening 90-minute session the track was still wet at the start of running.

The early running was made by Rosberg, who set a time of 1:55.102 which kept him at the top of the order as the half hour mark arrived. Ferrari’s Sebastian Vettel slotted into second behind the Mercedes driver with a time of 1:56.522 that put him six hundredths of a second clear of team-mate Kimi Raikkoinen, but both were then demoted by Ricciardo who posted a lap of 1:56.039.

Hamilton then emerged and with his first flyer his usurped the Australian with a time of 1:55.693. Ricciardo was still finding time on track, however, and he soon regained P2 with a lap of 1:55.592.

Rosberg's second run on intermediate tyres, 40 minutes into the session, lowered the benchmark considerably and a lap of 1:53.989 put him 1.2s clear of Kvyat and Ricciardo. Sainz improved to fourth with a lap of 1:55.667, while Vettel slotted in behind Hamilton with his best time of 1:55.710.

Max Verstappen was seventh fastest for Toro Rossoe, while Räikkönen, who had encountered brake difficulties after his first runs, finished eighth. McLaren's Fernando Alonso was ninth ahead of Valtteri Bottas of Williams, who gave his pit crew a scare when he slithered out of position on his approach to his pit box, making his mechanics jump to avoid being hit by his sliding car. 

Heavy rain began to fall ahead of the afternoon session and intitially race control decreed that the start of the session would be delayed indefinitely due to trackside marshals being unable to work "due to the proximity of lightning". 

The conditions worsened, however, and just short of an hour after the session had been set to start the session was cancelled as no medical helicopter operations were possible for the following two hours. 

2015 US Grand Prix – Free Practice 1
1 Nico Rosberg Mercedes 1:53.989 7
2 Daniil Kvyat Red Bull Racing 1:55.224 +1.235 6
3 Daniel Ricciardo Red Bull Racing 1:55.592 +1.603 10
4 Carlos Sainz Toro Rosso 1:55.667 +1.678 8
5 Lewis Hamilton Mercedes 1:55.693 +1.704 4
6 Sebastian Vettel Ferrari 1:55.710 +1.721 10
7 Max Verstappen Toro Rosso 1:55.969 +1.980 7
8 Kimi Raikkonen Ferrari 1:56.326 +2.337 10
9 Fernando Alonso McLaren 1:56.839 +2.850 18
10 Valtteri Bottas Williams 1:56.866 +2.877 15
11 Felipe Massa Williams 1:57.081 +3.092 13
12 Sergio Perez Force India 1:57.139 +3.150 4
13 Jenson Button McLaren 1:57.495 +3.506 20
14 Nico Hulkenberg Force India 1:57.518 +3.529 4
15 Romain Grosjean Team Lotus 1:58.319 +4.330 10
16 Raffaele Marciello Sauber 1:59.431 +5.442 19
17 Marcus Ericsson Sauber 1:59.743 +5.754 18
18 Alexander Rossi Manor 2:01.154 +7.165 11
19 Will Stevens Manor 2:01.907 +7.918 13
20 Pastor Maldonado Team Lotus 1