The session started on a damp track after earlier showers, but quickly dried, allowing drivers to switch from intermediate to slick tyres. Nevertheless, there was no shortage of spinners and off-track moments as everyone tested the limits of adhesion.
Kimi Raikkonen ended up fourth, just a tenth down on Ferrari team mate Vettel, though the red cars were barely quicker than the Red Bulls of Daniel Ricciardo and Max Verstappen, who notably - unlike their Mercedes and Ferrari rivals - did not use the faster, supersoft Pirelli tyre.
Rosberg, boosted by his newly inked two-year contract extension, was marginally quicker than Hamilton on the soft tyres, 1m 22.801s to 1m 23.078s. But on the supersoft rubber the world champion went ahead with 1m 21.347s to 1m 21.584s.
It was a good session for McLaren, with Fernando Alonso seventh on 1m 23.935s and Jenson Button eighth on 1m 23.961s, both using softs, as Romain Grosjean used supersofts on his Haas to take ninth on 1m 24.013s ahead of Force India’s supersoft-shod Sergio Perez on 1m 24.073s.
The track surface prompted plenty of action as drivers explored - and often exceeded - the available grip. Turn 4 caught out Hamilton and Verstappen; Turn 5 saw Alonso and Force India’s Nico Hulkenberg go off and Button spin; Charles Leclerc spun his Haas in Turn 6 before setting the 16th best time of 1m 25.181s; Hamilton went off in Turn 8 and Vettel in Turn 9; Turn 11 saw Leclerc and Raikkonen running wide; Vettel spun in Turn 12 as Alonso and Sauber’s Marcus Ericsson went off there; and Turn 14 saw Alonso spin and Esteban Ocon go off in his Renault.
A clearer picture of form should emerge in second practice this afternoon, when everybody runs the same rubber, but Mercedes have had a good start to the “flawless” weekend that team boss Toto Wolff says they will need if they are going to win here.