Ahead of today's opening session, the air temperature is 25 degrees C, while the track temperature is 26 degrees. As forecast it is wet. In fact, the rain has stopped, but it will be back.

Unlike previous weekends, we should see a fair amount of running today because we are expecting similar conditions for much of the weekend. Indeed, we are expecting some serious storms later today and tomorrow.

Only one reserve driver on duty this morning and that's at Sauber - celebrating its 400th GP - where Raffaele Marciello who is in Felipe Nasr's car.

"We'll install on a set of full wets," Hulkenberg is told, setting the mood for the morning.

The lights go green and Ericsson gets things underway. He is subsequently followed by Raikkonen.

As more and more drivers head out, including Vettel, Massa, Bottas and, local hero, Rossi, all navigate the Circuit of the Americas gingerly not wishing to make any costly, early mistakes.

Not that we're going to see them but tyre options are medium and soft. There are two DRS zones, the first on the back straight between T11 and T12, the second on the pit straight.

Ricciardo is the only driver to head out on Inters, as Ericsson pits and also switches to the green-banded rubber.

"Those full wet tyres put out around 65 litres of water per second at 300kph," reveals Pirelli. "With the amount of standing water, that's quite useful."

"There are some really big bumps on the straight," complains Ericsson before posting the first time of the day, 2:01.702. Whilst his rivals head back to the pits, the Sauber driver stays out and improves to 2:00.996.

Marciello is the second driver to post a time, the Italian stopping the clock at 2:01.855 as teammate Ericsson posts 1:59.743.

Despite a huge wobble, Rosberg goes quickest with a 55.102, the German aware that this weekend is one of the most important of his career.

As more drivers switch to the Inters, a spin for Massa, wo is on full wets, in T9, confirms how tricky conditions remain.

Raikkonen goes second (56.884), ahead of Button, Ericsson, Vettel and Marciello.

Raikkonen is told to pit, "we possibly have a problem in braking," the Finn is warned.

Unhappy with his car's handling, Button complains of bumps on the back straight. "That bump is seriously bad," he says, "it's like a jump".

The Briton is running the 'usual' engine this weekend, whilst teammate Alonso is running the updated version. In the hope that it proves a major step forward, Button has admitted that he hopes to be annihilated by his Spanish teammate this weekend.

Vettel goes second 956.522) but is soon leapfrogged by Ricciardo (56.039), all 11 drivers to have posted times thus far having done so on the Inters.

As Hamilton goes second (55.693), despite a very scrappy lap, Grosjean warns that his brake pedal is vibrating when he brakes. "Is that normal," he enquires. "We can see what you describe but we can't explain it," is the encouraging response.

Kvyat posts 55.224 to go second, ahead of Ricciardo and Hamilton.

With 54:40 remaining, all but Massa and Maldonado have posted times.

With an eye on whether now is the time to switch to slicks, Ricciardo reports that "the track is pretty greasy, not a whole lot of standing water. If I stayed out I'd have chewed up the tyres".

In the Williams garage the crew need to jump out of the way as Bottas overestimates the amount of grip as he slides into his slot. The Finn subsequently apologises.

In the Ferrari garage the crew forms a wall as the engineers try to resolve an issue on Raikkonen's car.

Rosberg ups the ante with a 53.989, the German now 1.235s clear of second-placed Kvyat.

At 'half-time', it's: Rosberg, Kvyat, Ricciardo, Sainz, Hamilton, Verstappen, Vettel, Raikkonen, Perez and Alonso. Maldonado, who has a gearbox problem, remains the only driver yet to post a time.