Red Bull's Sergio Perez finished fastest in the opening practice session for this weekend's Monaco Grand Prix.
Perez saw off Ferrari's Carlos Sainz by 0.119 seconds, with Max Verstappen third.
After predicting on Wednesday that Red Bull will be hard to beat this weekend, Lewis Hamilton finished fifth in his Mercedes, half a second behind.
What did you make of FP1? 🤔#MonacoGP 🇲🇨 #F1 pic.twitter.com/j5ciqCZocY — Formula 1 (@F1) May 20, 2021
Formula One is back in Monte Carlo after last year's race was pulled from the sport's Covid-hit schedule.
It marked the first time since 1954 that the sport's grandest race did not feature on the calendar.
The two-mile course has played to Red Bull's strengths in recent seasons, and Perez, in front of thousands of fans, delivered in the first running as the team lived up to their pre-race billing.
Perez set his fastest lap on the speedier soft rubber, with championship rivals Verstappen and Hamilton both on the slower, but more durable medium tyres.
😘 the apex. pic.twitter.com/topzPQ55v0 — Mercedes-AMG PETRONAS F1 Team (@MercedesAMGF1) May 20, 2021
Pierre Gasly finished fourth for AlphaTauri, with Hamilton's team-mate Valtteri Bottas sixth in the other Mercedes.
Double world champion Fernando Alonso became the first driver to fall victim to the narrow confines of Monte Carlo when he carried too much speed through Rascasse and hit the barrier.
The Alpine drive broke his front wing in the accident, and was forced to limp back to the pits for repairs.
Fernando Alonso took some front wing damage, he's now back in the pits We were briefly under yellow flags, but we are now back under way#MonacoGP 🇲🇨 #F1 pic.twitter.com/yOvHUT2jPt — Formula 1 (@F1) May 20, 2021
Sainz also nudged the barriers at the swimming pool complex but escaped without significant damage to his Ferrari.
His team-mate Charles Leclerc, who hails from Monaco, completed only four laps after he suffered a gearbox failure.
Elsewhere, Lando Norris, who earlier this week signed a new long-term deal with McLaren, finished seventh, 0.749 secs off the pace and one spot ahead of Aston Martin's Sebastian Vettel.
The second track action of the day gets under way at 3pm.
