Behind the leading duo, Max Verstappen recovered from a spell in the garage to take third place, a tenth and a half back of Vettel, while Valtteri Bottas and Daniel Ricciardo were fourth and fifth and the only other drivers to get within half a second of Hamilton’s benchmark.
In a surprisingly orderly session, nobody damaged their cars over the hugely unpopular kerb on the exit to the Swimming Pool, and only Romain Grosjean went off properly after locking his Haas’ brakes at Ste Devote, before spin-turning back into business.
The only other incidents were mild as Vettel, Verstappen, Kevin Magnussen and Pascal Wehrlein all ran over the chicane after overshooting their braking, while Jolyon Palmer ran wide at Mirabeau.
But there was on-track action throughout, partly to get as much track time as possible, and partly because the lack of degradation meant that drivers could still go fast on Pirelli tyres that were more than 20 laps old - even the ultrasofts.
However, one of the performances of the session came on the less popular supersoft rubber, as Daniil Kvyat finished an excellent sixth for Toro Rosso. That pushed Kimi Raikkonen, whose Ferrari was spotted kissing the barriers several times at the Swimming Pool, down to seventh, ahead of the Force Indias of Sergio Perez and Esteban Ocon, who sandwiched the other Toro Rosso of Carlos Sainz in ninth.
On his return to the McLaren cockpit, Jenson Button made a strong return and looked like he had never been away. Focusing primarily on longer runs and getting acclimated with the MCL32, Button pushed team mate Stoffel Vandoorne hard even when running soft tyres to the Belgian’s supersofts.
Button ended up 14th, two places back from Vandoorne, having accumulated 35 laps, which was 32 laps more than either Renault’s Nico Hulkenberg or Sauber’s Marcus Ericsson managed as both hit technical issues and failed to set timed laps.