Sebastian Vettel created the unexpected, but tantalising prospect of a Ferrari taking pole position for Sunday's Monaco Grand Prix when he outpaced both Mercedes men in Saturday morning's final free practice.
The four-time world champion, who is the only non-Mercedes driver to have won a race this year, was fastest round the barrier-lined streets of the Mediterranean principality in a best lap time of one minute and 16.143 seconds.
This lifted him two-tenths clear of fellow-German Nico Rosberg in the leading Mercedes and left him half a second ahead of championship leader Briton Lewis Hamilton, who was third fastest in his Mercedes.
Australian Daniel Ricciardo, Vettel's former team-mate, was fourth in his Red Bull, ahead of Spaniard Carlos Sainz of Toro Rosso, Finn Kimi Raikkonen in the second Ferrari and Russian Daniil Kvyat in the second Red Bull.
Briton Jenson Button demonstrated that the troubled McLaren-Honda team may end their pointless start to the season by clocking the eighth best time ahead of Dutch teenager Jos Verstappen in the second Toro Rosso and Frenchman Romain Grosjean, who was 10th for Lotus.
The session was run in cool, dry conditions and was interrupted only once for a red flag when Raikkonen lost control of his car at Ste Devote, smacking the left rear wheel into the barriers.
Hamilton had been fastest for most of the session until the final minutes when all the leading drivers went out on the new 'super soft' tyres introduced by Pirelli this weekend.
He complained that he was unable to improve his time because "something is not right with the car." His team told him they believed the tyre pressures were too high, a factor that may have caused him to experience handling difficulties as he seeks to land his first pole position for the classic 78-laps race.
Ferrari have not won a Monaco Grand Prix since seven-time champion German Michael Schumacher triumphed in 1991.