Carlos Sainz set the early pace at the Circuit de Monaco ahead of Fernando Alonso and Lewis Hamilton in the heavily-revised Mercedes.
The Ferrari driver set the benchmark of 1:13.372 to lead his compatriot by over three tenths of a second, and Hamilton a further three tenths back.
Sergio Perez finished P4 and was the lead Red Bull in a busy session, with teammate Max Verstappen infuriated by an ill-handling RB19 down in P6.
The championship-leading pair were separated by the second Ferrari of Charles Leclerc, who got away with some close calls at his home venue.
Lando Norris was P7 for McLaren, whilst the second Mercedes of George Russell was also struggling for rear grip in the session, finishing in P15.
Red flags were deployed twice in the session. Once for Nico Hulkenberg's tag with the barrier at the Nouvelle Chicane, and a second for Alex Albon's crash into the outside wall at Sainte Devote with less than three minutes to go.
Check out the full results from the opening free practice session below.
Result Free practice 1 - Monaco
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