Pirelli motorsport director Paul Hembery expects the majority of F1 teams to opt for a one-stop strategy in Sunday's United States Grand Prix, but admitted that the rate at which the surface of the new Circuit of the Americas evolves could change their thinking.

Pirelli returned to F1 racing in the United States for the first time since 1990, when the Italian tyres were last seen in a grand prix on the streets of Phoenix, and the teams got their very first chance to familiarise themselves with the hard and medium compound rubber on offer during Friday's two free practice sessions on the brand new circuit. Although cooler than expected temperatures added to the problems of a slippery surface, the drivers were able to get some extra running under their belts as Pirelli provided an additional set of the hard tyres.

With the teams relying purely on simulation data prior to arriving in Austin, there was plenty of work for them to do to analyse tyre behaviour on each compound under a wide variety of race conditions. Primarily, the work consisted of understanding the amount of wear and degradation on both of the compounds with full and empty fuel loads, in order to replicate the different qualifying and race conditions that will be seen over the course of the weekend. The teams also had the opportunity to try out different set-ups and see how they interacted with the tyres on this unique new circuit.

In the more representative conditions of the afternoon, the teams also moved on to the medium compound and more detailed tyre assessment took place that will provide crucial information when it comes to formulating race strategy for Sunday. With the asphalt in Austin being brand new, there was a high degree of track evolution resulting in the lap times getting quicker and quicker as more rubber was laid down, and this will continue to be the case throughout the rest of the weekend.

"On paper, we could say maybe it's a conservative choice, but it's normal for Pirelli as well," Lotus team boss Eric Boullier admitted, "It's a new track, new tarmac, so no racing before. Everything has to be built up. We could see a lot of track evolution over these two days, so that obviously has to be taken into consideration for the rest of the weekend, especially for qualifying and the race. Our engineers are still analysing, with Pirelli, the wear and the degradation and these kind of parameters, so I think it's just a choice which was done and we have to deal with it. We cannot complain or do anything."

Sauber's Giampaolo Dall'Ara admitted that the tyres had been the biggest challenge for the team to deal with in practice, while Kamui Kobayashi conceded that the rest of the weekend could be a struggle.

"I must say I have big warm up problems with both compounds," the Japanese driver reported, "It just doesn't work in qualifying if you need ten laps to get the tyres to work."

Hembery later admitted to Autosport that, based on the data from Friday, Pirelli would have brought the two softest compounds in its range, but maintained that the decision to provide the hard and medium rubber was the right one at the time.

"Today was all about familiarisation for the teams on what is the only brand new track of the year," he commented, "With weather conditions being cooler than expected, the initial data suggests that both wear and degradation is very low on both compounds, which leads us to think that we will see just one stop on Sunday, but we will have to look at all the numbers in more detail tonight to have a clearer picture.

"Our data simulation work has proved to be extremely accurate, with both the compounds performing exactly as we expected, although we also have to take into account the high degree of track evolution that could certainly affect the strategy on Sunday. We're seeing a performance gap of around half a second per lap between the two compounds, which gives the teams plenty of scope to try to gain an advantage through the timing of their pit stops. Nonetheless, as this is a completely new circuit with a number of potentially unknown factors, we deliberately opted for a conservative tyre choice here."

Mercedes' Norbert Haug, meanwhile, admitted that there could be no complaints laid at Pirelli's door given the step into the unknown that the latest USGP venue provided.

"We have to have an understanding for Pirelli in this case," the German insisted, "Of course, it's easy to say if you would have brought softer tyres, but I think they just didn't have enough data about the circuit, so they didn't exactly know how challenging this track would be. They went on the safe side and I think that's understandable."