Ahead of today's opening session, the air temperature is 27 degrees C, while the track temperature is 48 degrees. It is bright and sunny.

Of course, heading into the weekend, other than the future plans of Hamilton, Leclerc, Mercedes and Ferrari, the big topic concerns upgrades, particularly those at Mercedes.

As expected, the German team has ditched the zero sidepods concept, however, the actual design of the basic tub means that the team couldn't go as far as it wanted to.

Nonetheless, the upgrade is vital if the Silver Arrows are to keep Hamilton on board.

In terms of upgrades, Red Bull has a new Rear Wing and Front Suspension, while Ferrari has a Rear Corner and Rear Wing. As for Mercedes - deep breath - there's a new Front Suspension, Floor Fences, Sidepod Inlet, Coke/Engine Cover, Rear Wing and Rear Corner.

Alpine also has a whole host of updates, including a new Front Suspension, Sidepod Inlet, Floor Edge, Rear Wing and Beam Wing, while McLaren has revised Floor Fences, Front Corner, Rear Corner, Rear Wing and Beam Wing.

At Alfa Romeo there's a new Coke/Engine Cover, Floor Edge, Floor Body, Rear Suspension, Rear Corner, Rear Wing, Beam Wing and Cooling Louvres.

Aston Martin has a new Front Suspension, Front Corner, Rear Suspension, Rear Corner and Rear Wing, while Haas has a new Front Wing and Front Suspension.

At AlphaTauri there's a new Floor Body, Floor Fences, Floor Edge, Diffuser, Sidepod Inlet, Coke/Engine Cover, Rear Wing and Rear Wing Endplate, while Williams has a new Rear Wing, Beam Wing, Front Suspension, Front Suspension and Front Corner.

Of course, due to the very nature of the Monaco circuit, many of these are circuit specific.

The lights go green and Perez leads the way, followed by Bottas, de Vries, Tsunoda, Zhou and Stroll.

In no time at all there are 20 drivers on track, all but Norris and the Ferraris on hards.

"The clutch is almost impossible out of the pits," complains Verstappen.

Alonso posts a 20.403, but Hamilton crosses the line moments later at 20.045.

Norris crosses the line at 18.618, with Sainz going second ahead of Tsunoda and Leclerc.

As ever it is fast and frantic, with the order constantly changing.

"Car is bottoming a lot," reports Verstappen.

Norris, Leclerc and Sainz, the three medium runners lead the way.

As Sainz kisses the barriers at the Swimming Pool exit, Verstappen is complaining again. "The car in high speed places is unbelievably difficult to drive."

Meanwhile, a 15.931 sees Leclerc go quickest, while Hulkenberg (hards) improves to third with a 16.326.

No sooner has Sainz gone second again than Hamilton splits the Ferrari pair with a 16.196.

"The rears are too hot," reports Hamilton.

As Alonso goes second (15.668), despite his various troubles, Verstappen posts a 15.796 to go third.

"The rear is bottoming out massively," reports, well... you know who.

"I'm locking the rears into the hairpin," reports Russell, "it's tricky." The Briton, currently 18th, subsequently pits.

A 14.401 sees Sainz go top.

"I touched the wall a little bit," says Stroll of a 'kiss' at Rascasse, "but the car's ok."

After 22 minutes the Ferraris top the timesheets, but both are on mediums, with (hard-shod) Alonso just 0.413s off their pace.

After 25 minutes just 5 drivers are on track, as the rest pit for changes and to study the data.

"Something's wrong," reports Sargeant as he slowly makes his way back to the pits. "No drive," he adds.

Having switched to mediums, Hamilton goes quickest with a 14.035. Albon, also on mediums, goes sixth with a 14.731.

"It's a little bit better," says Verstappen of the changes made to his car, "but its'; still doing the same things on the bumps. Really not good."

"I can't get the tyres to work," says Russell, "I don't have the confidence. It's just sliding."

Hulkenberg has shed his left-rear tyre, though initially it is not clear why. Replay shows him spinning at the Nouveau Chicane and clouting the barrier opposite.

The session is red-flagged.

With 21 minutes remaining, it's: Hamilton, Alonso, Sainz, Verstappen, Norris, Leclerc, Albon, Perez, Tsunoda and Bottas.

The session resumes, and in no time at all there are a dozen drivers on track.

Switching to mediums, Alonso closes to within 0.064s of pace-setter Hamilton.

As Perez improves to third with a 14.227, Alonso goes quickest with a 13.907.

In the Red Bull garage there is frantic activity on Verstappen's car.

A 13.690 sees Sainz go top, as Verstappen and Hamilton appear to think it's Sunday as they battle over the same piece of tarmac, both missing the chicane in the process.

A clearly frustrated Verstappen throws his hand up in despair, the world champion not at ease with his car.

"These guys are incredible," complains Alonso of Perez who is on the racing line on a cool down lap. "I will stay on the racing line on my slow laps," he vows. The Mexican insists that he could not move.

Still 15th, Russell continues to struggle with his car while Leclerc is pushing like it is Q3.

The session is red-flagged again when Albon crashes at the exit of Ste Devote. Replay shows the youngster simply lost the rear end.

The session will not resume.

Sainz is quickest, ahead of Alonso, Hamilton, Perez, Leclerc, Verstappen, Norris, Ocon, Stroll and Albon.

Bottas is eleventh, ahead of Magnussen, Tsunoda, Gasly, Russell, de Vries, Piastri, Sargeant, Zhou and Hulkenberg, whose early crash meant he was the only driver not to try the medium rubber.

Check out our Friday gallery from Monaco here.