Ahead of the first practice session of the weekend, the air temperature is 19 degrees C, while the track temperature is 23 degrees. It is wet following an earlier shower. The likelihood of further rain during the session is estimated at 20%, which seems somewhat conservative seeing as the sky is totally grey with not a hint of a break in sight. Indeed, the track is declared wet.

Other than Brendon Hartley, who is in Pierre Gasly's car as the Frenchman chases the Super Formula title, Charles Leclerc and Sean Gelael are driving the Sauber of Pascal Wehrlein and the Toro Rosso of Daniil Kvyat respectively.

And talking of Hartley, the youngster, who has yet to turn the wheel of the Toro Rosso, is likely to start from the back of the grid after the team opted to change his engine. Max Verstappen, who in the moments before the session announced a new three-year contract extension, is likely to take a similar hit.

The lights go green and Vandoorne and Vettel lead the way, followed by Ericsson, Leclerc, Gelael and Grosjean. Hartley is another early riser.

Unsurprisingly, Grosjean reports that the outside of T1. Raikkonen reports "big bumps" on the back straight.

Bottas posts the first time of the weekend, the Finn stopping the clock at 50.529, as teammate Hamilton is warned that the kerbs are very slippery.

Onboard footage reveals a number of damp patches on the track along with a number of unwelcoming looking bumps which appear to have attracted the damp.

Ricciardo posts 50.730, Verstappen 50.314 and Hamilton 50.146.

15 minutes into the session, Hartley and Ricciardo are the only drivers on track. The Australian spins at T9 after running slightly wide. "Everything's fine," he reports.

Moments later, Gelael also has a spin, albeit at T15. The youngster subsequently posts 2:16.088.

The cameras pick up on Hamilton in conversation with his brother Nicolas and team boss Toto Wolff at the back of the Mercedes garage.

Ricciardo improves to third (50.494) as Ocon spins at T19.

Vandoorne is warned to heed the example of others, to avoid those kerbs and take care.

Verstappen goes quickest with a 47.922, though Inters remain the flavour of the moment.

26 minutes in and with just 11 names on the board Vettel heads out.

The German, on his first flying lap, goes second, just 0.350s off Verstappen's pace. Teammate Raikkonen posts 51.117 to go 9th.

Conditions are clearly improving, as Leclerc runs wide without penalty, while Massa improves to second with a 48.171. That said, we await the first driver to switch to slicks.

At which point Vettel complains: "The track is too dry". The German subsequently emerging on a set of supers.

Masa goes quickest with a 48.272 as Raikkonen is the second driver to switch to slicks.

All eyes on Vettel as he goes quickest in the first two sectors. He finally crosses the line at 45.960, leaving Massa only 1.732s in his wake, having made a mistake in the final corner.

"It's too wet," says Vettel, who only moments before had complained it's too wet.

After 38 minutes, all but Ericsson and Alonso have posted times.

Despite going quickest in S2, a couple of wobbles see Raikkonen abort his lap and dive into the pits.

Masa is the first non-Ferrari driver to try the slicks and the Brazilian is soon reporting that it is still too slippery, particularly in the final sector.

At 'half-time', it's: Vettel, Massa, Verstappen, Vandoorne, Sainz, Ocon, Stroll, Hulkenberg, Hamilton and Ricciardo. Alonso is the only driver not to post a time. Indeed, he isn't even in his car.

Despite the enthusiasm of the fans, the show has come to a halt and all 20 drivers remain in their garages. Vettel summed it up, too dry and too wet. With a long weekend ahead, not to mention Mexico next week, nobody wants to take any unnecessary risks.

Finally, Ricciardo heads out on supersofts. The Australian subsequently posts 40.297, 5.6s quicker than anything we've seen before.

It appears to take a minute or two for it to sink in, but eventually Hartley heads out on ultras as does Vandoorne, while Hulkenberg is on softs.

Ricciardo improves to 39.366 as Stroll and Raikkonen head out.

Hartley goes second with a 42.055 but is soon displaced when Hulkenberg posts 40.920.

Vandoorne complains of very low revs in 6th but is told that Honda is happy. The Belgian subsequently pits.

A 40.207 sees Raikkonen go second, 0.841s off Ricciardo's pace.

Despite going quickest in the first two sectors, Verstappen isn't happy, warning of a power issue. Nonetheless, he goes quickest with a 38.158.

All eyes on Hamilton, who, like the Bulls, is on the supersofts. He posts 36.935 to take the top spot.

Another unhappy driver is Vandoorne who complains: "I've locked up, it is shaking too much in sixth gear".

Bottas goes third with a 38.272, 1.337s off his teammate's pace.

Vettel complains of vibrations.

Bottas improves to second with a 36.979 as Hamilton consolidates his top spot with a 36.335.

A big, big lock-up for Bottas in T1 leaves the Finn with a left-front flat-spot, which causes the car to vibrate visibly. He subsequently pits... much to the relief of his rear wing.

Hartley complains that his helmet is lifting at high speed however, he is advised that there isn't the necessary time to make the relevant adjustments.

Meanwhile, Vettel has split the Mercedes duo with a 36.928, albeit 0.5s off Hamilton's pace. Verstappen is fourth, ahead of Massa, Raikkonen and the Pink Panthers.

Massa warns that it is "raining a little bit". Vettel echoes the Brazilian's weather forecast.

Lots of work on Ricciardo's car - the Australian taking an early lunch - but the team insists there are no problems and the work is purely set-up related.

Vandoorne improves to eighth with a 37.846, splitting the Force Indias which are the only drivers on softs.

Mercedes being very generous with the flo-vis on Hamilton's car, the front wing getting extra special dollops.

Check out our Friday gallery from Austin, here.