Lewis Hamilton edged out Red Bull's Max Verstappen by just 0.059 seconds to head first practice at the Monaco Grand Prix.
Hamilton's Mercedes team-mate Valtteri Bottas was 0.072secs off the pace as the top three drivers were closely matched around the street circuit.
Ferrari's Charles Leclerc was fourth, 0.361secs off the pace but 0.356secs ahead of team-mate Sebastian Vettel.
Mercedes and other teams carried tributes to the late Niki Lauda on their cars.
And Vettel was wearing a helmet modelled on the one the Austrian, who died on Monday, wore when he raced for Ferrari and won two of his three world titles in the 1970s.
A session of near misses
A number of drivers had 'moments' as they explored the limits of grip at one of the most demanding tracks on the calendar.
Williams driver Robert Kubica came the closest to a crash when he lost control on the exit of Casino Square. He slid down the track but somehow managed to avoid a crash, his front wing just nudging the wall on the inside as he came to rest halfway down the straight towards Mirabeau.
Pierre Gasly also had a near-miss when he got his Red Bull very sideways at the exit of the first Swimming Pool chicane, the fastest corner on the track, but just managed to rescue the car from hitting the wall.
And Kubica's team-mate George Russell reported that he had hit the wall but he got away without damaging the car.
Toro Rosso's Daniil Kvyat, McLaren's Lando Norris, Hamilton and Racing Point's Lance Stroll all went off into the run-off areas at Turn One, while Verstappen ran wide at Mirabeau.
Mercedes looking strong
Mercedes have struggled at Monaco for the last two seasons, but they have started this weekend in impressive form. Hamilton and Bottas were the pace setters from the very beginning of the session.
Verstappen was able to more or less match them but his runs were all a little later than the comparable Mercedes runs, so the Dutchman may have benefited from a slightly grippier track each time.
Behind the Ferraris, Red Bull's Pierre Gasly was sixth quickest, a second slower than Verstappen and just 0.038secs quicker than Renault's Nico Hulkenberg, who pipped Haas driver Kevin Magnussen, Alfa Romeo's Antonio Giovinazzi and the second Renault of Daniel Ricciardo.
Norris was 15th in the McLaren, his team-mate Carlos Sainz consigned to the garage with an electrical problem. Sainz was able to get on to the track only for one 'installation' lap right at the end of the session but the team said that he should be 'OK' for second practice.
His fellow British rookie Russell was 17th, 0.399secs ahead of Kubica. Anglo-Thai Alexander Albon was 14th in the Toro Rosso, a couple of places and 0.096secs behind team-mate Daniil Kvyat.