The FIA sporting regulations give Pirelli the option to provide tyres for Friday running, if the company wants to log more mileage. They can be used in either session.
The rarely-used rule states: “At certain events one additional specification of dry weather tyre may be made available to all teams for evaluation purposes following a recommendation to the FIA from the supplier.
“Teams will be informed about such an additional specification at least one week before the start of the relevant event.”
The tyres to be used in Austin will be the definitive 2020 specification, which is to be finalised after a development test to be conducted by Mercedes, Ferrari and Red Bull in Barcelona on October 7th-8th.
The extra Austin running, which will involve only one compound, will give all teams a head-start on understanding the latest construction.
A more extensive test, involving a range of compounds, will take place over three days in Abu Dhabi, following the last race of the season. After that teams will have to make their choices for the early races of 2020.
“We will supply the best option to all the teams in Austin,” said Pirelli F1 boss Mario Isola. “Two sets per car. This an additional opportunity to test the 2020 product.
“We are obliged to supply two sets of the same compound. We will identify a compound which is similar to one of the three selected for Austin, to make a proper comparison. The Barcelona test is a proper development test. Austin is let’s say, a validation.
“The selection for the race is C2, C3, C4. The development tyre, we still have to decide, but it will probably be the  C3.”