Max Verstappen recovered from a troubling start to the Monaco Grand Prix weekend to narrowly top Ferrari duo Charles Leclerc and Carlos Sainz in second practice.
Verstappen, whose Red Bull team have dominated the opening five races of the season, endured a frustrating opening session on Friday as he could only finish sixth amid struggles to get to grips with his RB19 around Formula 1's most famous circuit.
However, a much-improved display from the world championship leader saw him produce a lap of 1:12.462 to finish six hundredths of a second clear of Leclerc, who will be seeking a third successive pole at his home circuit on Saturday.
Sainz had enjoyed a hugely-impressive day, topping Practice One and showing strong pace on hard tyres early in the later session, but his afternoon was ruined by a heavy shunt at the Swimming Pool Chicane.
Fernando Alonso, who has been open about his belief he can claim a first F1 win for 10 years this weekend, was a little over 0.2s off Verstappen's leading time for Aston Martin.
Lando Norris offered a reminder of his special talent as he took advantage of McLaren's strong performance in slow corners to finish fifth.
That put him a place ahead of Lewis Hamilton, who appeared to settle more quickly in Mercedes' significantly upgraded W14 than team-mate George Russell, who could only manage 12th after finishing 16th earlier on Friday.
Verstappen's Red Bull team-mate Sergio Perez, who is also the Dutchman's nearest challenger at the top of the drivers' standings, failed to live up to the reputation he has earned for delivering at street circuits, as he finished seventh.
There had been lots of talk heading into the weekend about the unique nature of the Circuit de Monaco creating a chance for Red Bull's rivals to end the RB19's early-season domination, and such predictions looked to be accurate when Verstappen struggled in the opening session.
However, after some set-up changes during the 150 minutes between the sessions, the Dutchman was unsurprisingly right back into the mix.
While it was certainly an improved showing, the fact that Verstappen needed to complete an additional qualifying simulation - once the Ferraris had finished theirs - on an improving surface, was evidence that Red Bull are in a real battle to keep their 2023 winning streak going on Sunday.
"FP1 was quite tricky," Verstappen said. "I was not really happy with the ride of the car on the kerbs. FP2 was already a lot better, the car was a lot more competitive.
"Compared to Ferrari, we are still lacking on the general ride of the car - so how it handles the kerbs, the bumps, the drops in camber. So it's still something we need to work on for tomorrow.
"They are very, very close and knowing once we all go to the limit in qualifying, we need a bit more to stay ahead of them."
Sainz had delivered one of his most impressive days of the season as he outperformed Leclerc for much of it, but was left disappointed after ending the session in the garage.
The Spaniard hit the barrier on entry to the Swimming Pool Chicane, breaking his suspension and consequently leaving him powerless to prevent his car from slamming into the barrier on the exit.
The question is now whether Sainz can regain sufficient confidence to deliver the "all-out" style his team-mate says will be required to top Verstappen on Saturday.
Meanwhile, having expressed quiet confidence in their ability to compete for victory in the build-up, Aston Martin appeared to be a step behind Verstappen and the Ferraris.
Alonso is seeking an elusive 33rd F1 victory, but will need to find more speed on Saturday if he is to improve on four podiums in the opening five races of the season.
Along with excitement around the possibility of Red Bull being challenged for victory, the other big mystery going into practice had been how Mercedes' heavily-updated car would perform.
All eyes had been on the Mercedes garage since the team began setting up on Thursday morning, with the Silver Arrows finally moving on from the 'zero-sidepod' concept they have pursued since the start of last season.
Photos of the new bodywork, which is accompanied by a new floor and front suspension, emerged on Thursday, but it was only once the cars hit the track on Friday that real assessments could begin to be made.
Hamilton, who had previously admitted he was "counting the days" until the upgrades became available, appeared to enjoy his new machinery and backed up a third-placed finish in the opening session with a respectable sixth.
"I've generally had an amazing day," Hamilton said. "I really enjoyed driving today.
"I want to say a big, big thank you to everyone back in the factory because to build and design and develop a car is not an easy thing and everyone has put so many hours of hard work to have us here today.
"I'm glad we were able to keep it on track for them and I think we got a lot of data. It's not the place to ultimately test an upgrade but the car was generally feeling good."
Russell struggled amid several set-up changes during first practice, and despite improving between sessions, ultimately ended 12th.
"We need to try and figure some stuff out overnight," Russell said. "There are definitely some positive signs to take from the session, definitely improved from FP1, it's never easy around this place."
The Formula 1 season continues with the Monaco GP this weekend. Watch qualifying at 3pm on Saturday with lights out at 2pm on Sunday. Get Sky Sports