Ahead of today's opening session the air temperature is 30 degrees C, while the track temperature is 54 degrees.
First, the updates:
Mercedes has a new front wing endplate and front brake cooling, while Red Bull has updated it steering and front brakes cooling.
McLaren has similarly updated its rear brake cooling, steering, cooling louvres and mirror stay while Alpine and AlphaTauri have only upgraded their steering.
Williams has updated its steering and front brake cooling, while Haas has updated its steering.
No changes to the Ferrari, Aston Martin or Alfa Romeo.
The lights go green and Verstappen is first out, followed by Albon, Russell, Sainz and Perez. All but the Spaniard, who is on mediums, are on hards.
In no time at all there are 18 drivers on track, Latifi and Hamilton being the no-shows.
Bottas overshoots the Nouveau Chicane and has to take to the escape road.
Of the first wave, Leclerc goes quickest (17.993), ahead of Norris, Stroll, Ricciardo, Alonso and Magnussen.
Hamilton heads out and immediately goes fifth with a 20.072.
As is ever the case here, due to the fact the lap is so short the times are constantly changing, so much so that in the early stages it is almost impossible to keep track.
A 16.582 sees Verstappen go top, but Leclerc immediately responds with a 16.157.
Sainz complains about his ride, the wobble in his voice emphasising the porpoising issue which was expected to return here.
Sainz goes top with a 16.020 on the mediums, but is leapfrogged by his similarly-shod teammate who posts 15.882.
"There's an incredible amount of bouncing," reports Hamilton, as the on-board confirms the problem.
On the hards, Verstappen splits the Ferraris with a 15.883, just one-thousandth of a second off the pace.
Ste Devote sees its first off of the weekend, and it's Leclerc, moments after improving to 15.714.
Verstappen crosses the line at 15.327.
As Verstappen, Leclerc and Sainz continue to trade fastest times, Ricciardo goes fourth, ahead of Perez, Norris, Hamilton, Gasly, Vettel and Russell.
After 18 minutes there's still no sign of Latifi.
Schumacher reports a potential gearbox issue, the German subsequently grinding to a halt at the pitlane entrance.
As the marshals move the stricken Haas, the session is red flagged.
The session resumes but it is a couple of minutes before an engine fires up, it is the Mercedes in Stroll's Aston Martin. He is followed by Hamilton, Ocon and Latifi.
Albon, currently 13th (17.511) is the first to switch to softs.
"The bouncing's crazy, mate," reports Hamilton.
Vettel switches to the red-banded rubber as Stroll reports he has no front grip.
As Albon goes eighth (16.452), Ricciardo goes third (15.700) only to be demoted when Norris goes top with a 15.301, on the mediums.
Ricciardo (twice) and Gasly are among the drivers missing the apex at the Nouveau Chicane.
Norris consolidates his top spot with a 15.056.
Vettel fails to improve whilst Bottas' car is up on its stands, the Finn having completed just 2 laps.
Ricciardo stops the clock at 15.241 to make it a McLaren 1-2, but still, no doubt, short of Zak's expectations.
A 15.387 sees Vettel improve to fourth.
Gasly, on mediums, splits the McLarens with a 15.150.
Russell goes quickest in S1, maintaining a strong pace in the second sector. At the line the youngster posts 15.211 to go third, on the mediums.
A suspected MGU-K failure for Schumacher.
Currently tenth, Perez complains of a lack of grip with his mediums, as similarly-shod teammate, Verstappen, crosses the line at 14.712 to go top.
"Turn 5 is all about the bouncing," reports Albon, "I can't turn the car because the wheels are up in the air so much."
A 14.900 sees Perez make it a Red Bull 1-2.
Sainz, still on mediums, goes quickest in S1, and again in S2. However, traffic in the final sector sees him abort the lap.
The traffic being the result of yellows which were waved after Verstappen went off at Ste Devote.
Leclerc splits the Bulls with a 14.746.
Early days, but both Alpine drivers are off the pace, Alonso currently 16th and Ocon 19th.
Magnussen becomes the latest to make use of the escape road at Ste Devote.
Leclerc goes quickest in S1, and with PBs in S2 and S3 crosses the line at 14.531 goes quickest by 0.181s.
"That flat-spot is too big, box now," Magnussen is told.
Having gone quickest again in S1, Leclerc subsequently slows down, before winding it up again for another hot lap.
"These tyres are destroyed," warns Albon.
A great cloud of smoke fills the air as Perez locks-up big time at Ste Devote.
As before, Leclerc sets a blistering pace in S1, only to back off again.
"It's so bouncy out here man, I'm ******* losing my mind," says Hamilton. Maybe it's time to, ahem, "save the engine".
"Need elbow pads in this car, it's bouncing so much," he adds. The Briton is currently 10th and his teammate 8th.
The session ends. Leclerc is quickest, ahead of Perez, Sainz, Verstappen, Norris, Gasly, Ricciardo, Russell, Vettel and Hamilton.
Tsunoda is eleventh, ahead of Stroll, Alonso, Magnussen, Albon, Ocon, Zhou, Latifi, Schumacher and Bottas.
Early days, but it is clear that a number of drivers are struggling, not least the Mercedes pair, while others are clearly keeping their powder dry.
Then again, only a handful of drivers have tried the softs, though this will change in FP2.
Check out our Friday gallery from Monaco, here.
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