As Vettel shaded his team mate by less than a tenth of a second, Rosberg trailed by almost half a second, with Hamilton setting a near identical time to the championship leader. Best of the rest was Valtteri Bottas in the Williams.
Newcomers Haas again looked strong with Romain Grosjean sixth and Esteban Gutierrez eighth, split by the Red Bull of Daniil Kvyat. Williams’ Felipe Massa, running a new nose, and Red Bull’s Daniel Ricciardo completed the top ten.
For a while things were as normal - if you can accept that on the soft Pirelli tyres Rosberg and Hamilton each recorded exactly the same time - 1m 33.150s.
A little later in the session, when the track might be argued to have rubbered in more, Raikkonen showed the first sign of what was to come when he recorded 1m 32.863s on similar rubber, setting the fastest two sector times but failing to match Rosberg in the third.
On the supersoft Pirellis Rosberg then went fastest again on 1m 32.104s as Hamilton, ahead on both opening sectors, fell back ever slightly in the third to record 1m 33.160s - 0.056s down.
But then Raikkonen was back, and on the red-sided tyres the Finn underlined Ferrari’s superiority with 1m 31.723s before Vettel bettered that with 1m 31.683s.
That might just have shaded his team mate by 0.040s, but it left Rosberg 0.421s down and Hamilton 0.477s.
It was an odd session, with some people in places you wouldn’t have expected them to be.
Marcus Ericsson jumped to 11th for Sauber, who were in so much trouble yesterday that one of the Manors was ahead of them.
McLaren, so quick yesterday, were 13th and 14th on the supersofts, with the hugely impressive Stoffel Vandoorne just 0.040s down on Jenson Button despite staying in the garage for all but the final 18 minutes because of an oil leak.
And the Toro Rossos were only 15th and 16th after Max Verstappen and Carlos Sainz had respectively complained of battery drainage and puncture problems. But they only ran the soft-compound tyres. Likewise, the Force Indias weren’t quick either, in 18th and 19th.
So what does it all mean? We’ll find out in qualifying, when we’ll also see if Mercedes were sandbagging in this session or really do face a significant challenge from the red cars.