Ahead of today's opening session, the air temperature is 12 degrees C, while the track temperature is 18 degrees. It is somewhat cool, OK nippy even, and overcast, slightly foggy even.
Unlike Austin, there are no reserve drivers on duty this morning, while the cars of Perez, Massa, Wehrlein and Magnussen are all running the Halo device as testing continues.
The lights go green and the Gs get it underway, Grosjean and Gutierrez, they are followed by the Renault duo.
The circuit is very dusty and not at all grippy, as Wehrlein demonstrates as he runs wide.
Following its issue in Austin, indeed dating back to Bahrain, Haas is trying a new brake material.
In less than four minutes, everyone has been out, one of the last being Perez who gets a warm welcome from the crowd - if not the weather.
Sparks aplenty from Verstappen's car which appears to be running very low.
Vettel and Perez both report lots of dust.
Having completed the requisite installation lap all the drivers have headed back to the comfort, and warmth, of their garages.
After ten minutes of nothing, Ocon is the first driver to post a time, the Frenchman posting 1:28.842.
A big lock up for Alonso, who is switching between front wings this morning, the 'old' and a new one the team is trying. Nonetheless, the Spaniard posts a 26.398.
As Alonso improves to 25.933, Palmer and Button head out.
Alonso keeps chipping away, the Spaniard now down to 24.933.
"The main story so far is low grip," says Pirelli. "Just a question of running as much as possible to lay rubber down and generate as much heat as possible."
Running thus far has been on softs and mediums, however, Sainz, Kvyat and Hulkenberg are on supers.
Bottas goes quickest with a 24.337, ahead of Vettel (24.896), Alonso and Sainz.
It's business as usual as Rosberg posts 22.748 and Hamilton, who loses time in S2, 23.802.
Hulkenberg reports an issue with his DRS claiming that he cannot activate it anywhere, he is told to use it only where it is allowed.
Hamilton improves to 21.936 and his teammate 22.395.
Verstappen goes third (22.877), ahead of his teammate, but is demoted when Bottas improves to 22.837.
Perez delights the crowd when he posts 22.570 to go third, Gutierrez, currently 18th, doesn't appear to be stirring similar support.
The session is red flagged when Nasr loses part of his front wing after running over a kerb. Still running the Halo, the Brazilian is blissfully unaware. "Is something wrong," he asks "Is the car OK, I heard a strange noise."
While the replay shows him running wide, his front wing doesn't appear to hit the kerb.
Marshals take to the track to clear the debris.
"There's quite a mess on track," Ricciardo is told, "Nasr's front wing just exploded, basically."
As drivers are told that they may use their spare set of tyres for another seven minutes, Pirelli reveals: "So far, only five drivers have tried supersoft, Perez fastest of them."
At 10:42 the session resumes.
Verstappen's car is pushed into its garage, the left-rear brake smoking heavily. Even a blast from a fire extinguisher doesn't appear to help.
Wehrlein is the only driver yet to post a time.
Close up of Verstappen's car shows some burning damage around the brake drum to the carbon fibre. Indeed, this could be the end of the youngster's session. Indeed, the damage is to the rear brakes on both sides.
Around half-time, it's: Hamilton, Rosberg, Bottas, Masa, Verstappen, Raikkonen, Ricciardo, Vettel and Sainz.
Check out our Friday gallery from Mexico City, here.
Hamilton, on mediums, consolidates his top spot with a 21.527, while Rosberg posts 21.846.
All that time, Verstappen was in the car as his mechanics checked his brakes, but he now climbs from the Red Bull knowing that he has a problem.
As Rosberg closes to within 0.146s of his teammate, Gutierrez, now on softs, improves to 13th with a 23.419.
"As expected, we're seeing a lot of track evolution," confirms Pirelli. "Hamilton continues his leadership of this session; this time on the medium tyre."
Indeed, the Briton serves notice of his intensions this weekend by banging in a 20.914, thereby putting 0.639s worth of daylight between himself and Rosberg.
A 21.347 sees Rosberg (supers) split the Mercedes duo with Hulkenberg posting 21.607 moments later.
Bottas (softs) goes quickest in S1 however he fails to maintain the pace and remains sixth.
Having run wide in T1 and spun, Kvyat claims his rears weren't ready. He concludes the dialogue by signing off with a "speak to you later guys".
Told to watch out for Hulkenberg who is on a hot lap, Kvyat responds: "I'll let him by, I'm a good guy you know."
Vettel improves to third (21.407) on the softs, the German subsequently going second with a 20.993, just 0.079s off Hamilton's best.
Ocon is told to ignore his brake alarm.
Ricciardo returns to his pit and while there is no sign of smoke his crew are quick to whip off the wheel in order to cool his brakes.
"Be careful with the OK button," Vettel is told, "you're pressing it in the last sector." "Ok, no damage done," comes the reply. "I get the message."
Raikkonen improves to third with a 21.072, making it a Ferrari 2-3.
"There is a marshall standing outside the barrier at T7," reports Button.
You should have been here in the old days Jenson, it was marshals, fans and dogs... with no barriers.
"Top 3 times have been set on three different tyres," says Pirelli, "1) medium 2) soft 3) supersoft. Maybe not the order you would immediately expect."
"Use DRS, use DRS," Hulkenberg is told. "I did," says the German. "Both straights," comes the weary reply.
"Is everything okay with the car's behind?" asks Masa, another driver concerned at the continuing lack of grip and his brakes.
"I need to get out of the traffic," complains Raikkonen.
The Finn is subsequently the latest in a long line of drivers to lock-up, as Perez is told that there is rain at T3.
As the session ends, Button is told to back off after complaining of vibrations.
Hamilton is quickest, ahead of Vettel, Raikkonen, Perez, Hulkenberg, Bottas, Rosberg, Ricciardo, Massa and Kvyat.
Grosjean is eleventh, ahead of Sainz, Ericsson, Verstappen, Gutierrez, Alonso, Nasr, Button, Magnussen, Ocon, Palmer and Wehrlein.
Check out our Friday gallery from Mexico City, here.