Champions Mercedes' worst fears were confirmed on Saturday morning when championship leader Sebastian Vettel topped the times for Ferrari in final practice for this weekend's Monaco Grand Prix.
The four-time champion German clocked a best lap in one minute and 12.395 seconds, 0.345 seconds ahead of his team-mate Kimi Raikkonen, the 2007 champion, as the 'scuderia' established their supremacy ahead of qualifying later Saturday.
Vettel, winner of two of this year's five races so far, is aiming to enlarge his six-point lead ahead of Lewis Hamilton of Mercedes in the title race by claiming Ferrari's first victory in the classic Mediterranean principality showpiece since 2001.
Finn Valtteri Bottas was third-fastest for Mercedes, but three-time champion Hamilton was left behind in fifth, eight-tenths adrift of Vettel, in a session without major interruptions.
Hamilton was fastest in Thursday's opening practice but fell behind in the second and, amid concerns that the team's long-wheel base car has given them a distinct disadvantage, failed to recover significantly on Saturday.
Frenchman Esteban Ocon crashed his Force India into the barriers at the Swimming Pool exit in the closing minutes, without interrupting the flow of traffic such was the efficiency of the marshals, and Australian Daniel Ricciardo parked his Red Bull at Sainte-Devote corner after a brake-by-wire problem.
These minor incidents brought out brief use of the Virtual Safety Car (VSC), which interfered with the movement of a busy circuit of cars in the closing seconds.
Dutch teenager Max Verstappen was fourth for Red Bull ahead of Hamilton, Ricciardo and the Toro Rosso pairing of the ever-improving Spaniard Carlos Sainz and Russian Daniil Kvyat.
Dane Kevin Magnussen of Haas and Belgian Stoffel Vandoorne of McLaren Honda completed the top 10.
Briton Jenson Button, the 2009 champion, making his return from retirement, was 12th for McLaren Honda. However, he is set for a 15-place grid penalty after his team discovered an engine problem, meaning he will start from the back of the grid.
After using two new engine components the limit of four per season per car was exceeded.
He is a replacement this weekend for two-time champion Spaniard Fernando Alonso, who is racing this weekend in the Indianapolis 500.
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