Daniel Ricciardo headed team-mate Max Verstappen to a Red Bull one-two in both practice sessions at the Monaco Grand Prix.
The Australian saw off the Dutchman by 0.194 seconds in the second session on Thursday afternoon as both enjoyed a comfortable margin over their rivals.
Ferrari's Sebastian Vettel was third, one place ahead of Lewis Hamilton.
Hamilton was 0.123secs behind but had a messy session and appeared to have more pace in his Mercedes.
The world champion, who has a 17-point lead over Vettel after five races, made a few errors that compromised his overall lap time.
But there will be questions as to whether Mercedes are struggling in Monaco for the second year in succession after he said over the radio that something did not feel right while he was doing a race-simulation run on the super-soft tyre, the hardest of the three compounds available this weekend.
"It's been an OK day, we didn't suffer any damage which is a good thing," said Hamilton.
"I said yesterday the Red Bulls were going to be quick which they were today. As expected, we struggled a little bit more. The car felt good in some places, in others it felt bad. So we have got some things to work on, but we're not completely in the dark; we're in a much better place than we were last year.
"We're closer to Red Bull and Ferrari than I expected, but we're still a few tenths off. So we've got some ground to cover and pick up if we want to be in the fight for the win."
Running in race trim on the hyper-soft, Vettel appeared to have a pace advantage over Hamilton of about 0.3secs, although practice times are not always an accurate measure of actual form.
"It was a decent day," said Vettel. "I am not that happy yet in the second and last sector. I trust in the car so I can play around but I am sliding a bit too much so I think there is something we can do better.
"I don't think we got the best out of the tyres when we were running due to traffic. For Saturday it will be very close."
It was Red Bull, though, who dominated the day, with both Ricciardo and Verstappen immediately getting to grips with the demanding track.
Verstappen appeared to have the advantage in second practice until Ricciardo edged clear as he strung together a series of laps on the hyper-softs a little later than Verstappen's run.
Red Bull have flattered to deceive in practice a number of times already this season, but there are signs that this weekend might be different.
Ricciardo famously lost victory in Monaco in 2016 when he was leading comfortably only for his team to fail to have his tyres ready at a pit stop, handing the lead to Hamilton.
Ricciardo said this week that he felt Monaco still owed him a win, but added: "I've got to go and earn it. It's not going to happen without me putting the effort in. Maybe some circumstances will help me out but for sure I've still got to be prepared to put the effort in."
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Renault's Nico Hulkenberg was seventh fastest, ahead of the McLarens of Stoffel Vandoorne and Fernando Alonso, the latter's day compromised slightly by losing most of the first session because of a brake-by-wire issue.
Haas, who impressed at the last race in Spain with Kevin Magnussen finishing sixth, well clear of the other midfield runners, appeared to be struggling, the Dane 16th with team-mate Romain Grosjean two places further back.
There was only 0.5secs between Hulkenberg in seventh and Sauber's Charles Leclerc in 17th.
Behind the scenes, Ferrari are the subject of an investigation into the operation of the energy store in their hybrid system by governing body the FIA, who are concerned that it could be used in an unscrupulous way.
Despite its concerns, the FIA has no evidence it has been used in such a manner and insiders say the FIA, after an inquiry lasting more than a month, is moving towards a position where it is satisfied there is no chance the energy store can be used in a way it should not be.
And Vettel appeared unfazed by the episode when questioned after second practice.
"We heard about it but it is normal that every now and again you have something popping up and then there's rumours," he said.
"This time it's us. In four weeks' time probably someone else. Ultimately it is the FIA's job to look after everyone and we trust them as much as the other teams do to do their job."