Lewis Hamilton is playing catch-up in Monaco after Red Bull's Daniel Ricciardo completed a practice double at the principality.
Ricciardo was fastest in the opening running on Thursday before setting a record-breaking lap later in the day to cement Red Bull's status at the pre-race favourites.
The Australian finished both sessions ahead of his team-mate Max Verstappen, with Sebastian Vettel third in the order for Ferrari.
Hamilton, who holds a 17-point championship lead over Vettel ahead of Sunday's race, was fourth for Mercedes, more than half a second adrift of Ricciardo's blistering time of one minute and 11.841 seconds.
The champion arrived here fearing he would be off the pace of both Red Bull and Ferrari at the slow-speed circuit in which his Mercedes team have struggled at in recent years.
And the 33-year-old British driver will have work to do if he wants to challenge at the sharp end of the grid come qualifying on Saturday.
Hamilton's team-mate Valtteri Bottas was sixth in the order to emphasise the team's sluggish pace, with Kimi Raikkonen fifth for Ferrari.
Ferrari notoriously keep a little in reserve in practice and Vettel was 0.572 seconds adrift of Ricciardo's best effort.
But it is Red Bull who will head into the remainder of the weekend confident that they can secure their first pole position since Ricciardo stormed to pole here two years ago.
On a largely, and perhaps surprisingly incident-free day at a circuit where the smallest of errors are punished, the second session was suspended for 16 minutes after a drain cover worked its way loose.
Race director Charlie Whiting inspected the damage at Mirabeau before the drain was welded down, and the action was allowed to resume.
Fernando Alonso, the double world champion who missed most of the morning running with a brake issue, was absent from last year's race to take part in the Indianapolis 500.
But on his return to the famous Monte Carlo streets, he will be encouraged by his position of ninth with his McLaren team-mate Stoffel Vandoorne one place ahead of him.
Nico Hulkenberg was seventh for Renault, with team-mate Carlos Sainz completing the top 10.