Ahead of the weekend's sole practice session the air temperature is 25 degrees C, while the track temperature is 38 degrees.
In terms of updates, Red Bull has a new Floor Edge and Coke/Engine Cover, thereby continuing the steps previously taken, more efficient cooling can be attained with the geometric changes to minimise the louvre openings.
Mercedes has a new Front Wing, Front Suspension, Floor Edge, Sidepod Inlet, Coke/Engine Cover and Floor Fences. In terms of the floor edge, the additional vane element increases mass flow under forward floor, increasing vorticity shed from the fence system, increasing floor load.
McLaren has a new Front Wing, Front Suspension, Front Corner, Rear Corner and Beam Wing. The front brake duct furniture has been updated to complement the changes on the front wing and front suspension, resulting in overall improved flow characteristics. This is suitable for tracks with low front brake cooling demand, a reduced cooling front brake duct has been designed, improving overall aerodynamic load at the expense of front brake cooling.
Aston Martin has a new Front Wing, Front Wing Endplate, Coke/Engine Cover, Floor Body, Floor Edge and Diffuser as the bodywork and floor in combination improve the flowfield under the floor increasing the local load generated on the lower surface and hence performance.
Alpine has a new Floor Body, Floor Edge, Coke/Engine Cover and Rear wing, the bodywork having been reshaped to improve flow conditioning and to better interact with the floor and the rear of the car.
Stake has a new Front Wing, Front Suspension, Rear Suspension and Rear Corner, whereby the upper deflectors have been updated in combination with the top wishbone cover. Improved local flow and positive interaction with the updated component brings a small efficiency increase. In terms of the front wing, the updated geometries aim to improve the front tyre flow structures. This has a positive effect to the flow field further downstream on the car, improving both overall downforce of the car and the aero characteristics.
For its home race, Haas has a new Sidepod Inlet, Floor Body, Floor Fences, Floor Edge, Rear Corner, Coke/Engine Cover and Cooling Louvres, which, in combination with the new engine cover, mean that additional cooling louver options are available, which increase heat extraction and try to minimize the drag penalty.
Ferrari, Williams and RB have not brought any updates to Austin. Then again, with just the one practice session there is no real opportunity to test them.
Other than the Red Bull ride height saga, the main talking point heading into the session is the return of Liam Lawson, following RB's decision to drop Daniel Ricciardo.
As has become tradition a number of teams are running one-off liveries... until the next one-off livery.
The lights go green and Norris leads the way, followed by Alonso, Stroll, Hulkenberg and Ocon. As more drivers emerge almost all are on hards, bar Bottas who is on mediums.
Drivers are warned of a 15 kph headwind into Turn 1.
"You know what the state of play is this weekend," Verstappen is told as Colapinto gets a little out of shape and runs wide.
A 36.724 sees Leclerc set the early pace, ahead of Verstappen, Russell, Hamilton and Sainz.
Zhou, the last driver to emerge, is on softs.
As Russell reports that the track feels "very smooth", Norris goes second with a 36.725, only to be demoted when Sainz stops the clock at 36.140.
Russell subsequently goes top with a 35.860, however Verstappen responds with a 35.538.
Gasly spins at Turn 1 in an Alpine that looks remarkably like a McLaren but doesn't perform like one. "I lost it under braking," he admits.
Both Stake drivers continue with their own tyre strategy, Bottas on the mediums and Zhou on the softs.
A wild moment for Hamilton as he hits the bump at Turn 4 and spins. "You'll have to check the tyres and the floor," reports the Mercedes driver. "I had a moment there also," adds Russell.
Bottas improves to sixth (36.421) on his mediums, as Sainz raises the bar with a 35.231.
Russell spins as he exits Turn 1, bringing out the yellows.
Perez goes fifth as Tsunoda improves to sixth.
The Japanese driver complains of understeer and front locking as teammate Lawson improves to tenth with a 36.702.
A 34.966 sees Leclerc go top as Ferrari continues to set the early pace.
The wind also appears to be catching a number of drivers out, not least Norris who has a couple of offs.
Lawson reports an issue with low speed stability.
"A messy opening twenty minutes to our session," admits Mercedes.
With 32 minutes remaining, Verstappen switches to the softs, as does teammate Perez.
Verstappen bangs in a 33.855, 1.111s up on Leclerc, though Sainz - still on hards - responds with a 34.717.
As Perez goes third (34.717), posting exactly the same time as soft-shod Sainz - Verstappen reports that his tyres were a "little cold" at the start of the lap.
"The main struggle was the ride in the low speed," reports Perez... now, if only there was a cure for that.
Hamilton heads out again following his spin in Turn 4.
Perez improves to second, but remains 0.783s off his teammate's pace.
With 20 minutes remaining, the majority of drivers are sticking with their used hards - no pun intended.
"I have a problem guy, I have no power," says Zhou as he enters the pitlane.
"Can you give me some feedback as to how wide drivers are running exiting Turn 9, over that kerb," asks Russell. He is told that Verstappen, in particular, is right up to the limit.
On softs, Albon runs wide in Turn 6, kicking up a great cloud of dust. He subsequently goes off at Turn 12.
The Ferraris, Lawson, Stroll and Hamilton are next to switch to the red-banded rubber.
On the softs, Colapinto improves to fifth with a 35.248.
Hamilton splits the Red Bulls with a 34.314, 0.459s down on Verstappen.
Lawson goes third with a 34.443, but is demoted when Sainz goes quickest with a 33.602.
Alonso goes third, Gasly fifth and Ocon eighth.
A 34.618 sees Albon go seventh, as Russell runs wide in Turn 1.
"Missed pit entry," reports Piastri after making a mistake which sees the VSC momentarily deployed, much to the frustration of Leclerc.
Tsunoda goes fourth with a 34.313, while Russell can only manage seventh.
Replay shows Russell enjoying a little argy-bargy with Colapinto.
"Ride is definitely worse at this fuel load," reports Russell.
Piastri is given a black and white flag for failing to keep to the correct side of the bollard at the pit entry.
Quickest in the final sector, Leclerc goes second with a 33.623 as Hamilton consolidates fifth with a 33.963.
Norris goes fourth (33.868) and Piastri fifth (33.908).
"I don't have a second lap, right," asks Leclerc. "No you don't," he is told, this a result of that previous VSC.
Verstappen's early 33.855 has stood the test of time, while his teammate's failure to improve has resulted in him dropping to sixteenth.
The session ends. Sainz is quickest, ahead of Leclerc, Verstappen, Norris, Piastri, Hamilton, Russell, Magnussen, Alonso and Tsunoda.
Hulkenberg is eleventh, ahead of Gasly, Lawson, Albon, Stroll, Perez, Ocon, Bottas, Colapinto and Zhou.
A tricky session for almost everyone, the conditions suggesting that we could be in for some surprises this afternoon and tomorrow.
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