Ferrari's Sainz fastest in first Monaco Grand Prix practice
Seven-times world champion Lewis Hamilton set the third quickest lap, in a new-look Mercedes with significant upgrades to the side pods and suspension, with last year's winner Sergio Perez fourth for Red Bull.
Perez had warned before the start of the track action that Monaco was not Red Bull's strongest circuit, despite the champions having won all five races so far this season with four one-twos.
The Mexican identified the Ferrari drivers and Alonso as the biggest threats.
Sainz's best time of one minute and 13.372 seconds, on a beautifully sunny afternoon in the Mediterranean principality, was set on the medium tyres and with Ferrari president John Elkann watching.
His best was 0.338 quicker than Alonso's effort.
The session ended three minutes early when the red flags came out after Alex Albon crashed his Williams at the exit to the Sainte Devote first corner, wrecking the left side of his car.
Practice was also suspended with 22 minutes to go when Nico Hulkenberg clipped the barrier and spun his Haas at the Nouvelle Chicane after the tunnel.
Ferrari's Charles Leclerc, the home hero who will be chasing his third successive Monaco pole position on Saturday, lapped fifth fastest with Red Bull's double world champion Max Verstappen sixth and uncomfortable with his car's setup.
McLaren's Lando Norris was seventh fastest.
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