Daniel Ricciardo led a Red Bull one-two in the opening practice session for Formula 1's Monaco Grand Prix.
Ricciardo jumped to the top of the times in the final third of FP1 to displace team-mate Max Verstappen, who is under investigation for reversing back onto the track after an off at Ste Devote.
Lewis Hamilton was Red Bull's closest challenger in third, almost three tenths back for Mercedes, with Ferrari's Sebastian Vettel fourth but 0.9s in arrears.
Red Bull first moved to the top in the opening half hour by using the hypersoft tyres to displace the ultrasoft-shod Mercedes.
Valtteri Bottas had settled at the head of the field on a 1m14.347s after exchanging the place with team-mate Hamilton before Ricciardo beat him by more than two-tenths of a second.
Ricciardo and Verstappen then whittled the benchmark down, with Verstappen becoming the first to break the 1m13s barrier on a 1m12.941s.
That gave Red Bull a huge 1.2s gap over the Ferrari of Vettel, who moved into third on hypersofts, as the bulk of the track action stopped approaching the midpoint of the session.
Sergey Sirotkin was one of few outliers because the Williams driver had interrupted his earlier running when he clouted the pit wall earlier on entering the start-finish straight and deflated the right-rear tyre.
He jumped into the top 10 and stayed there as others improved to give Williams some early-weekend cheer.
At the head of the order Hamilton put Mercedes briefly back on top with his second lap on hypersofts, a 1m12.480s, but Verstappen beat that by a tenth soon after.
The Dutchman improved further to a 1m12.280s, before suffering a huge front-left lock-up into Ste Devote that put him down the escape road.
He was placed under investigation for failing to rejoin safely when he reversed on track with a Ferrari approaching the corner.
Ricciardo took a couple of attempts to take advantage after traffic in the final sector initially kept the Australian 0.056s adrift, but eventually beat him by 0.154s on a 1m12.126s.
That shuffled Hamilton to third and Vettel fourth in a Ferrari that is running pre-Spanish GP rear suspension configuration and has also come under intense scrutiny for its battery arrangement within the engine's energy recovery system.
Vettel finished just 0.025s ahead of team-mate Kimi Raikkonen, while Renault's Carlos Sainz Jr jumped to sixth with 20 minutes to go.
Valtteri Bottas's Mercedes was dumped to seventh as the Finn, wearing a Mika Hakkinen tribute helmet, struggled to post a truly competitive time on hypersofts.
McLaren had a tricky season, with Stoffel Vandoorne and Fernando Alonso ending up 15th and 17th respectively.
Alonso missed most of the session as McLaren had to change the floor after a brake-by-wire problem, before appearing late on and improving to 17th.
Spanish GP star Kevin Magnussen was slowest as he and Haas team-mate Romain Grosjean completed just 16 laps between them.
A late flier from Grosjean launched him into the top 10 behind Sergio Perez's Force India and ahead of Sirotkin.
|1||Daniel Ricciardo||Red Bull/Renault||1m12.126s||-||36|
|2||Max Verstappen||Red Bull/Renault||1m12.280s||0.154s||25|
|8||Sergio Perez||Force India/Mercedes||1m13.717s||1.591s||39|
|11||Esteban Ocon||Force India/Mercedes||1m14.000s||1.874s||39|
|12||Brendon Hartley||Toro Rosso/Honda||1m14.034s||1.908s||46|
|14||Pierre Gasly||Toro Rosso/Honda||1m14.240s||2.114s||38|