Max Verstappen laid down his marker for a crucial Monaco GP qualifying by setting the pace in final practice, although only just ahead of the in-form Ferraris who appear to be in the hunt for a shock pole position.
With F1 2021's formbook reset around the famous Monte Carlo streets to kick-off Saturday, Mercedes' Valtteri Bottas and Lewis Hamilton were only fourth and seventh with Carlos Sainz instead posing the closest challenge to Verstappen, who has topped every Practice Three session this season.
Verstappen finished fastest with a 1:11.294, 0.047s ahead of Sainz.
"Don't underestimate the importance of being P1 in free practice just before qualifying - it's just that extra bit of mental strength and power," said Sky F1's Nico Rosberg, a three-time Monaco winner.
Charles Leclerc was two tenths of a second further back in the other Ferrari, whose quickest times came on older tyres compared to Verstappen and so could be genuine challengers for their first pole since 2019.
Bottas was 0.471s off the pace and championship leader Hamilton a whopping 0.726s as both Mercedes struggled, according to Sky F1's Ted Kravitz, with balance issues in their cars.
"It looks to be game over for Mercedes," said Rosberg. "They just don't have the grip out there and can't get the tyres to work. They're really up against it."
Sergio Perez was fifth in the second Red Bull, with Lando Norris continuing his strong start to the weekend in sixth for McLaren. Team-mate Daniel Ricciardo was adrift again, down in 13th.
A fast and unpredictable final practice was marred by two big crashes, with Nicholas Latifi crashing his Williams at the exit of the swimming pool chicane and Mick Schumacher losing control of his Haas at Casino Square.
Schumacher, in particular, will be struggling to make the qualifying session due to the damage to his car.
There looks to be another very close battle for the top midfield slots ahead of one of the most exciting and important qualifying sessions of the season.
Kimi Raikkonen was a surprise eighth for Alfa Romeo, ahead of Pierre Gasly and Sebastian Vettel.
Surprisingly Alpine were down in 15th through Fernando Alonso and Esteban Ocon.