Ahead of today's opening -budget cap free -session, the air temperature is 24 degrees C, while the track temperature is 45 degrees. It is hot and sunny.
Upgrades this week see a new floor edge at Mercedes, new engine cover (cooling) at Red Bull and rear brake ducts, engine cover: slots, Gurney & cooling louvres for McLaren.
Alpha Tauri has a new front brake duct, engine cover and cooling louvres, while Aston Martin has a revised rear deflector, Williams new cooling louvres and Alfa Romeo a new floor (front geometry & cut-outs).
As was the case last week there are a number of young drivers on duty, with Nyck de Vries in the Mercedes, Liam Lawson in the AlphaTauri, Logan Sargeant in the Williams, Pietro Fittipaldi in the Haas and Jack Doohan in the Alpine
They replace George Russell, Yuki Tsunoda, Alex Albon, Kevin Magnussen and Esteban Ocon.
The lights go green Vettel leads the way, followed by Fittipaldi, de Vries, Zhou and Bottas.
Among the early risers is Perez, who, not unexpectedly, receives a warm welcome from the crowd.
Soon, all bar the Williams duo are on track.
All runners are currently on the hards.
Ricciardo sets an early benchmark, the Australian stopping the clock at 25.867. Norris goes second with a 26.001.
"I have no brakes," claims Lawson, "brakes are completely gone." This appearing to be a bit of a habit with the AlphaTauri.
As is so often the case, Verstappen heads out, and immediately goes quickest in all three sectors, crossing the line at 22.779.
However, Perez responds with a 22.702, much to the delight of the locals.
Bottas goes third (23.563) ahead of Ricciardo, Gasly, Sainz, Hamilton and Latifi.
Verstappen retakes the top spot with a 22.291.
Gasly improves to third with a 23.561, as Sainz goes fifth (23.746).
"De Vries is behind on an out lap," Sainz is told, however, the Spaniard mishears and thinks he is being warned about "debris".
Twenty minutes in and there is a lull, as only Sainz, Vettel, Lawson and Stroll remain on track.
Having only completed 2 laps, Leclerc is the only driver yet to post a time.
At which point the Monegasque heads out... on softs. Schumacher also makes the switch to the red-banded rubber, as does Bottas.
The Ferrari driver crosses the line at 21.546, 0.745s up on Verstappen's best on hards.
Schumacher goes second (21.952), just 0.406s down on Leclerc.
More and more drivers now switch to the C4s, as Leclerc improves to 20.753.
Norris goes second (21.120), ahead of Bottas, Schumacher and Zhou.
Perez goes third, the Mexican 0.389s down on Leclerc as Alonso goes second.
A spin for Verstappen at Turn 11, the world champion losing the rear after running over a kerb.
"It's unbelievable, I couldn't correct it," admits the Dutchman. "So little grip."
Meanwhile, a 20.707 sees Sainz go quickest.
Zhou has stopped at the end of the pitlane. "I can't upshift," he admits. "I've switched the engine off because it gets too hot."
However, having passed the white line that marks the end of the pitlane and start of the track, he has to be rescued by the marshals as opposed to his team.
As the Alfa Romeo is pushed back up the pitlane, Fittipaldi stops at Turn 3.
The session is red-flagged.
Clearly there was an issue with the car, for the youngster was ordered to stop the car in in uncertain terms.
The session resumes with 18 minutes remaining and first out is Vettel, followed by Sargeant, Lawson, Stroll and Schumacher.
For many, attention shifts to Sunday as the drivers switch back to hards, among them Verstappen.
Guenther Steiner admits that Magnussen may require a new engine.
Still on the hards, Hamilton improves to fifth (20.849), while Alonso goes sixth (20.899).
Hamilton causes problems for a number of drivers, including Ricciardo, in the stadium as he slows and moves from one side if the track to the other.
Offs for Lawson and Verstappen.
Alpine announces that Doohan's session is over, the youngster having stopped as a precaution as a "result of seeing anomalies in the PU data".
Lawson stops at Turn 13, smoke billowing from his left-front brake.
"Mate, what the **** is going on," he had asked the team, before being told to "stop the car".
The session is red-flagged again.
As the youngster climbs from the AlphaTauri flames are visible.
The session will not resume.
Sainz is quickest, ahead of Leclerc, Perez, Verstappen, Hamilton, Alonso, Bottas, Norris, Gasly and Vettel.
Ricciardo is eleventh, ahead of Zhou, Stroll, Schumacher, Latifi, Lawson, Sargeant, De Vries, Doohan and Fittipaldi.
Check out our Friday gallery from Mexico City here.
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