MONTE CARLO - Championship leader Sebastian Vettel threw down a gauntlet to his rivals in emphatic fashion on Thursday when he topped the times for Ferrari by almost half a second in the second free practice session for this weekend's Monaco Grand Prix.

On the tightest and most intricate circuit on the Formula One calendar, the four-time champion blitzed around the streets of the Mediterranean principality to clock a best lap in one minute and 12.720 seconds. The German's time was 0.487sec faster than the second fastest lap from Austrtalian Daniel Ricciardo of Red Bull who, in turn, was less than one-tenth of a second ahead of third-placed Finn, Kimi Raikkonen, in the second Ferrari. Russian Daniil Kvyat was fourth for Toro Rosso.

Briton Lewis Hamilton, who lies just six points behind Vettel in the drivers' title race and who was fastest in the opening morning session, was more than a second adrift down in eighth, but without clocking a clean hot lap on the ultra-soft tyres. His Finnish team-mate Valtteri Bottas was 10th, also without showing his true pace during a busy session interrupted by one red-flagged stoppage when Frenchman Esteban Ocon crashed into the barriers at the entry to Casino Square in his Force India car.

The Mercedes team expressed concern before the weekend began that their car, with the longest wheel-base of all, would struggle for agility around the demanding and technical corners. Briton Jenson Button, back from retirement to replace Spaniard Fernando Alonso who is racing in the Indianapolis 500, was 14th for McLaren Honda, one second and two places behind his team-mate Belgian Stoffel Vandoorne.