Nico Rosberg dominated the first practice session for the US Grand Prix in Austin, held in wet-but-drying weather conditions.
Rosberg led the field by 1.235s on intermediate tyres, ahead of the Red Bulls of Daniil Kvyat and Daniel Ricciardo.
Points leader Lewis Hamilton – who can win the world championship this weekend – was fifth quickest, 1.7s off Rosberg's pace and behind Toro Rosso's Carlos Sainz.
Story of the session
The 90-minute session started in wet conditions, although the showers had abated by the time the green flag flew. The air temperature was a 25C, the track surface was only one degree warmer, but the high humidity – 95 per cent – meant it was very slow to dry.
"Not too bad," was Marcus Ericsson's early report on the conditions, and Sauber's Swede was the first to complete a flying lap on intermediate tyres, a 2m01.702s. He broke the two-minute barrier on his sixth lap, a 1m59.743s.
Rosberg then leapt to the top of the times on 1m55.102s after a brief off at Turn 1.
Kimi Raikkonen went P2 in his Ferrari on 1m56.583s before being told he had to pit for a "potential problem with braking", before teammate Vettel beat him by 0.061s on 1m56.522s.
Daniel Ricciardo topped both Ferraris on 1m56.039s to go second fastest after the first 30 minutes, before Hamilton went P2 on his first flyer on 1m55.693s, half a second off Rosberg's pace. But Ricciardo fought back with 1m55.592s to regain second spot.
Daniil Kvyat went P4 on 1m56.244s, then progressed to P2 on 1m55.224s, just a tenth off Rosberg's then-fastest time. Toro Rosso's Carlos Sainz went fifth fastest on 1m56.398s.
Rosberg's second run on inters produced a 1m53.989s to put himself 1.2s clear of the opposition. Sainz improved to fourth on 1m55.667s, and Vettel improved his time to 1m55.710s for sixth.
Conditions improved enough for DRS to be enabled with 40 minutes to go, as the humidity tumbled and a drying line emerged.
Ricciardo aborted a slick run with 20 minutes remaining, as the skies brightened considerably towards the end, despite a few spots of rain in the increasing breeze.
After his brake issue was solved, Raikkonen went for a second run on inters late in the session, improving his time to 1m56.326s but staying eighth.
Max Verstappen was seventh fastest, ahead of Raikkonen, McLaren's Fernando Alonso and Valtteri Bottas of Williams.
Few incidents despite tricky conditions
Williams had a lively session: Felipe Massa spun his car at Turn 9 on his first flying lap, having to perform a reverse turn to rejoin: "I just lost the car on the kerb on the exit," he reported.
Bottas gave his pitcrew a scare when he locked up on the way into his pitbox, but they all managed to jump out of the way just in time.
"Sorry about the pitlane, there was a lot less grip than I thought," he admitted.
Drivers complained of the large bump that has emerged on the backstraight, Jenson Button referring to it as "like a jump".
Sauber's reserve driver Raffaele Marciello suffered a brief off at Turn 19, but he outpaced Ericsson by three tenths of a second.
Alonso had a late excursion into the Turn 1 run-off area after locking up, but rejoined.
Pastor Maldonado lost most of the session's running due to a gearbox issue after his installation lap.
|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 Jr.||Toro Rosso||1:55.667||+1.678||8|
|7||Max Verstappen||Toro Rosso||1:55.969||+1.980||7|
|12||Sergio Perez||Force India||1:57.139||+3.150||4|
|14||Nico Hulkenberg||Force India||1:57.518||+3.529||4|
|15||Romain Grosjean||Team Lotus||1:58.319||+4.330||10|
|20||Pastor Maldonado||Team Lotus||1|