Charles Leclerc capped an impressive return to F1 action for Ferrari by heading their second successive one-two in Practice Two at the Belgian GP.
Leclerc took over from P1 pacesetter Sebastian Vettel in the afternoon's running at Spa-Francorchamps and his best lap of 1:44.123s was a full 0.6s faster than his team-mate's alone.
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Mercedes, third with Valtteri Bottas and fourth with Lewis Hamilton, finished 0.8s adrift over F1's longest lap, with Red Bull more than one second off the pace.
"Today the Ferraris are the absolute benchmark and could be on Sunday - and also in Monza," said Toto Wolff, Mercedes' team principal, to Sky Sports F1.
Sergio Perez was an impressive fifth for Mercedes-powered Racing Point, although he ran into late trouble when the back of his car caught fire and he had to pull off the track.
Alex Albon finished 10th at the end of his first day for Red Bull but within 0.4s of Verstappen, with the Anglo-Thai driver's focus more on race set-up with his career at the team to start with a hefty grid penalty for engine changes on Sunday.
Verstappen, who does not have Honda's latest engine here, told Sky F1 he was running with reduced power during Friday as the team preserve engine mileage.
Is Ferrari's advantage really that large?
Hamilton had predicted on Friday that Ferrari might prove "formidable" this weekend, with the strength of the Italian team's car this year well-established as being its straight-line speed.
That advantage was most pronounced in Spa's mostly-flat-out first sector on Friday, with Mercedes' superior strength in the twister middle phase of the 7km lap not enough to cancel out Ferrari's early-lap advantage.
But Sky F1's Karun Chandhok suggested: "I don't think we've seen the best of Mercedes yet. Daniel Ricciardo's Renault was quicker through the first sector, for instance."
All six Mercedes-powered cars are running the Anglo-German squad's new engine this weekend and Racing Point took full advantage on a strong opening day when both Perez and Lance Stroll were consistently in the top 10.
But the late fire which stopped Perez will concern the team overnight, particularly if the cause was linked to the power unit.
Kimi Raikkonen was a strong seventh for Alfa Romeo during P2, with Stroll, Ricciardo and Albon rounding out the top 10.
Those latter three drivers all carry grid penalties into the weekend for exceeding engine limits, as does 11th-fastest Carlos Sainz in the McLaren.