Red Bull and Ferrari took the fight to Mercedes in the opening practice for the Singapore GP where Daniel Ricciardo set an all-time new lap record.
Although Mercedes set the early pace on the supersoft tyres, Red Bull were soon challenging at the front – although they were running the ultrasofts.
Max Verstappen went quickest with a 1:43.2 while Ricciardo lined up second.
As more drivers, including Vettel, put in ultrasoft tyres, Lewis Hamilton and Valtteri Bottas began to drop down the order.
The duo bounced back when they swapped to the ultras with Hamilton going quickest – a 1:42.9 – and Bottas third.
Neither Mercedes driver was able to put in a second flier on the ultras with Bottas reporting on his sixth lap on the purple-marked rubber that his tyres “were gone.”
Red Bull, though, returned to the fore as Ricciardo posted a 1:42.4 to finish FP1 quickest of all.
Vettel put in a late run to go second quickest, 0.109s down, while Verstappen was third, a further 0.012s off the pace.
Hamilton was fourth ahead of Sergio Perez, Bottas, and Kimi Raikkonen while Fernando Alonso finished P8.
Ahead of the expected McLaren-to-split-with-Honda-and-sign-Renault announcement, the Spaniard clocked a 1:43.7 but yelled “no, please no” when asked if he wanted another run after complaining about a lack of power.
Nico Hulkenberg and Daniil Kvyat completed the top ten.
Romain Grosjean, P15, and Antonio Giovinazzi, who filled in for Kevin Magnussen, only run the supersoft tyres throughout the 90-minute session.
Lance Stroll did not see the chequered flag as a gearbox problem meant his session ended early with 11 laps on the board.
1 Daniel Ricciardo Red Bull 1:42.489
2 Sebastian Vettel Ferrari 1:42.598 0.109
3 Max Verstappen Red Bull 1:42.610 0.121
4 Lewis Hamilton Mercedes 1:42.904 0.415
5 Sergio Perez Force India 1:43.423 0.934
6 Valtteri Bottas Mercedes 1:43.434 0.945
7 Kimi Raikkonen Ferrari 1:43.734 1.245
8 Fernando Alonso McLaren 1:43.759 1.270
9 Nico Hulkenberg Renault Sport 1:44.101 1.612
10 Daniil Kvyat Toro Rosso 1:44.220 1.731
11 Stoffel Vandoorne McLaren 1:44.340 1.851
12 Jolyon Palmer Renault Sport 1:44.961 2.472
13 Esteban Ocon Force India 1:45.053 2.564
14 Felipe Massa Williams 1:45.084 2.595
15 Romain Grosjean Haas 1:46.456 3.967
16 Antonio Giovinazzi Haas 1:46.782 4.293
17 Lance Stroll Williams 1:47.190 4.701
18 Sean Gelael Toro Rosso 1:47.570 5.081
19 Marcus Ericsson Sauber 1:47.699 5.210
20 Pascal Wehrlein Sauber 1:47.886 5.397