Ahead of today's opening practice session the air temperature is 16.2 degrees C, while the track temperature is 20.3 degrees. As expected it is overcast and though rain is not expected during this session we are expecting some in FP2. It is also quite windy.
Following heavy rain overnight, the track is a little damp in places.
There is only one reserve driver on duty this morning, Nicholas Latifi replacing Robert Kubica at Williams - possibly a portent of 2020 - the first of three more outings for the Canadian.
The lights go green, and Norris leads the way, followed by Hulkenberg, Raikkonen and Kvyat.
Interestingly, while Norris is on hards and Kvyat mediums, the others are on Inters, as are the next drivers to emerge. Latifi and Magnussen are on softs, the Canadian also sporting bijou aero rakes behind his front wheels.
"The steering is not straight," complains Raikkonen.
Hulkenberg is told to look after his Inters in the high-speed corners as he will probably need them tomorrow.
The install laps done, most pit in order to switch to slicks.
As drivers report that the track is more damp than anticipated, especially off the racing line, most opt to play it safe, with Hamilton not looking likely to appear anytime soon.
Fourteen minutes in and Hamilton finally heads out, the Briton reporting that "that track is dry". Nonetheless, he is on the Inters.
As Hamilton pits, Ricciardo heads out and when the Australian subsequently stops he is replaced by Vettel.
As Bottas prepares to head out again, it's a disjointed session, with drivers, aware of the weather predictions for the remainder of the weekend, unwilling to take too much out of there Inters, while also loathe to make the full switch to slicks.
"I wouldn't say it's a dry line," says Latifi, " but for the most part it is dry."
It's a real heart-in-mouth lap for Bottas who is hanging out the rear of the Mercedes in every corner. He posts a 21.887, while Hamilton can only manage 26.358.
Sainz goes second (23.119), ahead of Giovinazzi, Kvyat and Latifi.
"Rear end's all over the place," reports Hamilton, the Briton still managing to post a 21.127 and thereby displace Sainz who had just gone quickest with a 21.613.
Vettel, like Hamilton on mediums, posts 20.418 but is instantly demoted when Bottas stops the clock at 19.889. The Finn is told to "be careful on the kerbs".
No sooner has Hamilton gone quickest than Verstappen bangs in a 19.035, the Dutchman on the softs.
After thirty minutes, all but Norris and Ricciardo have posted times.
As Sainz reports graining to his fronts, Hamilton goes quickest with a 19.004.
The yellow flags make a brief appearance when Vettel goes wide in Turn 1 Gasly making a similar mistake at Turn 11 shortly after.
As Leclerc goes quickest (18.849), Verstappen looks set to raise the benchmark but runs wide in Turn 12 and has to abort the lap.
A big, big lock-up for Giovinazzi in Turn 1, the Italian needing to pit immediately due to the resultant flat-spot on his fronts.
"My left-front looks like cheese," reports Russell.
At the point at which the extra set of tyres have to be handed back, it's: Leclerc, Hamilton, Verstappen, Bottas, Vettel, Stroll, Hulkenberg, Perez, Albon and Kvyat.
It is seven or eight minutes before an engine fires up, it is the Mercedes of Latifi.
The Canadian, now sporting the softs, improves to 17th with a 21.699.
"I've picked up a little bit of understeer," reports Grosjean after running wide in Turn 11, which is where the FIA has located its track limits sensors this weekend. On softs, the Frenchman is only able to manage 18th (21.586).
On softs, Albon posts 17.949 to go quickest.
Stroll has a curious moment as he exits the stadium, the Canadian slamming into the barriers full sideways on. He is able to drive back to the pits but the Racing Point is sure to have incurred some damage.
"****, sorry," says the Racing Point driver.
While the session is red-flagged as the debris cleared from the track, the timing shows the session as "finished", despite there being 30 minutes remaining.
Meanwhile, following his earlier indiscretion at Turn 11, Grosjean is the first driver to have his time deleted.
The session resumes and there is a sizeable queue of drivers keen to get back to work.
Quickest in all three sectors, Hamilton goes quickest with a 17.327, as Vettel is seemingly in a battle with Verstappen and the Haas pair as they seek some clear track.
"There was a mess in the last corner," reports the German. Verstappen is also unhappy. "Sebastian, what the **** is he doing, everybody is waiting and he wants to go by, unbelievable."
Vettel goes fourth (18.701) on the softs, while Verstappen, like Hamilton on softs, goes second with a 17.472.
On the mediums, like his teammate, Leclerc goes second, just 0.19s off Hamilton's pace.
An 18.897 sees Gasly improve to eighth, the Frenchman still eyeing that second Red Bull seat in 2020.
With nine minutes remaining, Stroll is back on track, the damage to the Racing Point clearly not as bad as originally feared.
Leclerc is a staggering 14 km/h quicker than Hamilton through the speed-trap.
As Gasly report unease with the front of his car, teammate Kvyat is unhappy with the rear.
Taking a hard ride over the kerbs at Turn 1, Magnussen reports that his tyres are "starting to grain".
The session ends. Hamilton is quickest, ahead of Leclerc, Verstappen, Albon, Bottas, Vettel, Sainz, Gasly, Kvyat and Giovinazzi.
Hulkenberg is eleventh, ahead of Magnussen, Raikkonen, Norris, Ricciardo, Stroll, Perez, Grosjean, Russell and Kubica.
Check out our Friday gallery from Mexico City, here.