Sergio Perez blocked Charles Leclerc from whitewashing practice at the Monaco Grand Prixby setting the fastest time in FP3.
The Mexican went to the top of the time sheets by just 0.041s with a lap right at the death of the hour-long session to fire a warning shot across Ferrari’s bows ahead of qualifying.
Perez and Leclerc traded quickest times for the final 10 minutes of the session as they squeezed the final drops of performance from the soft Pirelli rubber.
By the time the checkered flag was waved, the pair had broken well clear of the their teammates, with Carlos Sainz third, but 0.37s off the pace. Max Verstappen was a further 0.035s behind the second Ferrari driver.
It sets up an intriguing crucial qualifying hour after Ferrari seemed set to have the run of the place on Friday night.
Pierre Gasly headed the midfield for AlphaTauri, but it was closely contested between the Frenchman and McLaren’s Lando Norris, the pair split by 0.016s and about three-quarters of a second behind Perez.
Lewis Hamilton was mystified by a 1.2s gap to the leaders early in the session – his car was still bouncing aggressively, sapping his confidence to hug the walls through the middle sector – and by the end of the hour he’d closed the gap to only 0.9s, setting Mercedes up for a midfield qualifying battle.
Kevin Magnussen followed for Haas 0.061s further back, with George Russell just 0.04s further adrift in ninth, though the Mercedes driver was aggrieved to have been blocked by Perez in an incident that will be investigated after the session.
Fernando Alonso completed the top 10 ahead of Yuki Tsunoda and Mick Schumacher.
Sebastian Vettel was 1.3s off the pace ahead of Valtteri Bottas for Alfa Romeo and Williams driver Alex Albon.
Daniel Ricciardo completed a session-high 29 laps as he attempted to make up for lost time after crashing in FP2, for which his team accepted the blame for setting up his car too aggressively, but the Australian was 1.6s off the leading pace after suffering from overheating brakes in the 80 degree F ambient temperature.
Esteban Ocon was 17th for Alpine ahead of the crashed Lance Stroll, who took too big a bite of the inside curb exiting the Swimming Pool and whacked his right-hand-side against the barrier. He broke his front wing but was able to limp back to pit lane to avoid a stoppage.
Zhou Guanyu finished the session 19th and under investigation for impeding Verstappen, with Nicholas Latifi 20th.