Ahead of today's opening session, the air temperature is 17.9 degrees C, while the track temperature is 23 degrees. It is bright and sunny, which is not what was expected. Indeed, it has been widely forecast that conditions today, tomorrow and Sunday would be similar to those experienced in Austin last Friday.
There are a number of reserve drivers on duty today, Nicholas Latifi at Racing Point, Antonio Giovinazzi at Sauber and Lando Norris at McLaren. They replace Ocon, Leclerc and Alonso.
Talking of Austin, race winner Raikkonen is running the new floor introduced last week, while Verstappen is running Red Bull's version. Teammates Vettel and Ricciardo are running the old-spec floors for comparisons purposes.
Courtesy of the altitude, not to mention last year's result, Red Bull is expecting to be very strong this weekend, possibly the Austrian outfit's last win of the season... and last ever with Renault.
Of course, that will comes as good news to a certain Mr Ricciardo.
At sister team Toro Rosso, Honda reveals that Gasly has a new new-spec power unit as there is concern at the assembly of the unit used last week - which means for the second successive race he will start from the back of the grid. Talking of Toro Rosso, this week Hartley has the updates used on Gasly's car and subsequently removed.
The lights go green and the McLaren duo are first out.
More and more drivers head out, and just three minutes into the session all but local hero Perez and the Mercedes pair have appeared. A mixture of all three tyre compounds in use.
Perez finally appears, and as he enters the stadium section gets the hero's welcome he gets every year.
The first flurry of activity out of the way, things go quiet.
After a few minutes of silence, Austin winner Raikkonen heads out again, his Ferrari sporting the hypersofts.
As the Finn posts the first time of the weekend (1:19.798), he is joined by teammate Vettel. The German subsequently posts 19.696, also on the hypers. Raikkonen reveals that he has flat-spotted his tyres and subsequently pits.
Vandoorne and Hartley head out.
Vandoorne is the third driver to post a time (21.561), albeit 1.86s off Vettel's pace.
On board footage reveals that just after a couple of laps, Vettel's fronts are showing signs of graining.
"Tyres are suffering, just very superficial," reports the German. However, a lap later he reports that "it's a waste of running like this".
"Pulling on the brake in T12 I get a lot of front locking," warns Hartley.
A 20.115 sees Perez go third, as Bottas finally heads out.
Latifi is also struggling for grip, a combination of a dusty track and lack of down force due to the thin air.
As Perez goes quickest (19.469) and Sainz second (19.523), Hamilton finally heads out. Like his teammate he is on the hypers.
Verstappen goes quickest (18.588) and Ricciardo second - both on the hypers - as Bottas goes third and Hamilton posts a cautionary 37.478.
Looking set to improve considerably, Verstappen's third sector is ruined by Giovinazzi, who twice gets in the way of the Red Bull driver. Meanwhile, Ricciardo goes quickest with a 17.932.
An 18.268 sees Hamilton go third, the leading 11 drivers all on hypers.
Sainz reports that his tyres are finished, demonstrating the point by taking a long run over the grass.
At 40 minutes, at which point the extra set of tyres have to be handed back, it's: Ricciardo, Verstappen, Hamilton, Bottas, Sainz, Perez, Vettel, Raikkonen, Hulkenberg and Giovinazzi. Gasly has yet to post a time, though he has completed two laps. The Frenchman's car back on its stand as the engine is changed.
"Brutal traffic, Williams (Sirotkin) almost run me off the road," reports Hartley.
In the Ferrari garage, Vettel appears to be having the new floor fitted to his car.
Verstappen is also unhappy at being blocked by Sirotkin, but his language is a little more colourful.
Following over ten minutes of inactivity, the sessions roars back to life again as Raikkonen, Giovinazzi, Hartley and Ericsson head out.
A spin for Raikkonen at T6, just moments after going quickest in S1. The Finn losing the rear of the Ferrari after missing the apex.
Now on ultras, Hamilton is around 0.5s down on his previous best. However, his S1 time was a PB.
Hartley improves to fifth with a 19.024 as Hamilton complains of "a lot of dips in power". The team advises the Briton that it will "look into it".
A PB is S1, is followed by a purple in S2, as Sainz posts an 18.117 to go fourth, just 0.185s off Ricciardo's pace.
Hartley is very, very unhappy at almost being forced off the track by someone or other. "He's an amateur!" moans the kiwi.
As Hamilton continues to suffer power issues, Hulkenberg improves to third with an 18.028.
Having had an 'issue' with Magnussen, a trouble-free lap sees Verstappen - still on hypers - go quickest in all three sectors, thereby raising the bar with a 16.656.
"Try to cool those tyres as best you can," Bottas is warned, "rears are raging hot."
Meanwhile, Sainz has leapfrogged his Renault teammate with a 17.926 to take third.
Verstappen warns that his brakes are pulling at the front. The Red Bull driver going quicker in the first two sectors before experiencing a poor final sector.
Now on ultras, Vettel reports that his fronts are flat-spotted following a lock-up.
Attention now shifts to Sunday afternoon as drivers focus on longer runs, most on the hypers, a few on the ultras.
While Grosjean describes his tyres are like being on wets, Hartley describes his as being like "bubble gum". A TV close-up shows the Haas shedding marbles in all directions.
Norris has to take drastic avoiding action to ensure he doesn't collect Latifi, both drivers - all drivers - really struggling for grip.
The session ends.
Verstappen is quickest, ahead of Ricciardo, Sainz, Hulkenberg, Hamilton, Bottas, Vettel, Raikkonen, Hartley and Latifi.
Perez is eleventh, ahead of Giovinazzi, Grosjean, Ericsson, Norris, Vandoorne, Magnussen, Sirotkin, Stroll and Gasly.
While Ferrari opted to concentrate on evaluating its updates, it's clear that tyres are going to be a major factor this weekend, and as the drivers struggle for grip, with few support races, they face further problem this afternoon and tomorrow.
Check out our Friday gallery from Mexico City, here.