Red Bull's Daniel Ricciardo sprung a surprise by comfortably heading the Mercedes drivers in second practice for the Monaco Grand Prix.
The Australian finished 0.606 seconds quicker than Lewis Hamilton, who earlier set the pace in first practice.
Hamilton's Mercedes team-mate and championship leader Nico Rosberg was third, a further 0.293secs adrift.
Red Bull's Max Verstappen was fourth, ahead of the Toro Rossos of Daniil Kvyat and Carlos Sainz.
Saturday's qualifying and Sunday's race are live on the BBC sport website and radio 5 live.
Ferrari sluggish, Vettel sloppy
The fastest Ferrari was only seventh, in Kimi Raikkonen's hands, ahead of the Force India of Sergio Perez.
Sebastian Vettel was only 14th when he spun at Mirabeau corner, hitting the barrier and damaging the rear wing of his Ferrari.
He later jumped up to ninth only to slide into the barrier at Ste Devote on his very next lap, although he got away without any damage.
Manor's Rio Haryanto was the only other man to crash in a session punctuated by drivers locking wheels and running wide at the first corner and chicane.
The Indonesian, whose long-term future at the team is in doubt over sponsorship concerns, lost control over the crest after the tunnel, spinning into the outside barrier, rebounding into the inside barrier and coming to rest in the escape road. He was unhurt.
Red Bull scintillating
Red Bull had been expected to feature close to the front in Monaco, especially as Ricciardo is using one of only two existing versions of an upgraded Renault engine which have been rushed forward to an early debut for this race.
But the size of his margin to Mercedes was surprising - and the engine did not account for anything like the 0.964secs gap between Ricciardo and Verstappen, who won the last race in Spain.
The new engine is said to be worth up to 0.5secs a lap on a more normal circuit, but only 0.2secs around Monaco, where power has less of an effect on lap time than anywhere else.
Red Bull's pace appeared genuine - Ricciardo was also the quickest driver on the race-simulation runs during the second part of the session, by a little over 0.1secs a lap on average from Rosberg.
But Mercedes always find extra pace on Saturdays and still must be considered strong favourites for the weekend.
Meanwhile, the McLarens' pace was disappointing after they came into the weekend predicting both drivers would be in contention for the top 10 on the grid and behind only Mercedes, Red Bull and Ferrari.
Jenson Button was 10th, 1.718secs off the pace, and team-mate Fernando Alonso 0.4secs adrift in 11th place after not setting a representative lap on the fastest 'ultra-soft' tyre.