Charles Leclerc saw off Lewis Hamilton as Ferrari dominated first practice for Sunday's Monaco Grand Prix.
The unique Monte Carlo layout had been expected to favour Ferrari's machinery, and so it proved with Leclerc leading the way.
Red Bull driver Max Verstappen finished third in the principality -- albeit half-a-second off the pace -- with championship leader Kimi Antonelli fourth.
George Russell, bidding to bounce back from his mechanical failure at the previous round in Canada which leaves him 43 points behind Antonelli in the world championship, was a distant fifth.
The one-hour session was suspended twice after Isack Hadjar crashed out in his Red Bull before Aston Martin's Fernando Alonso clipped the barrier on the exit of the tunnel.
Mercedes has won all five rounds so far, but, on the evidence of the opening running of the weekend, Ferrari appear primed to end the Silver Arrows' victorious streak.
The low-speed track plays into Ferrari's hands and Leclerc, who became the first Monégasque to win on the sport's famous streets two years ago, delivered an impressive turn of pace.
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Fresh from signing a new contract with Ferrari which is set to keep him with the Italian giants for at least another three years, Leclerc beat Hamilton, searching for his first win with the Scuderia, to top spot by 0.226 seconds.
Verstappen finished best of the rest, 0.513 seconds slower than Leclerc, with Antonelli, a winner of the last four grands prix, 0.559 sec behind. Russell finished one second back. Lando Norris, last year's winner here, was sixth for McLaren on the occasion of its 1,000th race.
Midway through the session, Hadjar lost control of his Red Bull through the high-speed Swimming Pool Chicane and ended up in the barrier.
Hadjar was unharmed in the crash but the force of the entry ripped the left-rear wheel off the Frenchman's car.
The red flag was deployed before being re-introduced in the closing minutes when Alonso broke his front wing after making contact with the barrier on entry to the Nouvelle Chicane.
Alonso finished 20th of the 22 runners. The second practice session starts at 4 p.m. BST.