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Pirelli has raised the United States Grand Prix minimum rear tyre pressures between qualifying and the race, meaning that tyre performance could be a step into the unknown for Formula 1 teams.

Before a race weekend Pirelli issues prescriptions that include minimum pressures and camber limits, and it reserves the right to make adjustments if necessary.

Usually that happens after Friday running, but because of the rain it had no dry tyre information until Saturday.

Due to parc ferme regulations Pirelli can't mandate a camber change after qualifying, and has to rely solely on adjusting rear tyre pressures from 20 to 21.5psi, in a bid to protect them after they suffered more than expected.

"We need to guarantee the integrity of our product, the system is just for that," Pirelli F1 boss Mario Isola told Autosport.

"When we have a wet Friday like it was here we collect data from FP3 and quali.

"We started to analyse data immediately after FP3, and we were in a position to give feedback after a few hours, that is the usual system.

"We had a higher severity on the rear tyres compared with simulations, so we had to raise the pressure.

"It's not very big, but it's a sensitive change. If it happens on Saturday, we don't change the camber, we just review the pressure."

Asked what the impact might be in the race Isola added: "It's not a big change in terms of balance.

"It's difficult to predict any effect also, because we don't have much data coming from the weekend. I didn't have any complaints or feedback from the teams."

After qualifying Mercedes boss Toto Wolff made it clear that rear tyres could be the key to the race, especially with Kimi Raikkonen on a different starting tyre to the Mercedes drivers and his Ferrari team-mate Sebastian Vettel.

"We've seen today in the long runs that the tyres are rear limited, so the car's rear limited," Wolff said.

"And that will be a topic in the race, how to make the tyres last. Because strategy wise it's not completely clear what the optimum strategy is."

Carlos Sainz Jr, who as the 11th-fastest qualifier has freedom to choose a harder tyre for the start than those ahead, expects tyres to be a "curve ball" in the race.

"There are some unknowns, especially because the long runs were done in very cool conditions, for the race we expect the sun out," he told Autosport.

"It will create a tricky scenario on tyres like we saw in Suzuka on Sunday, or Austria for example, where the track temperature came up on Sunday and we could see some blisters.

"With that curve ball thrown into the race we could have an advantage."