Dr Helmut Marko has admitted for the first time that Brendon Hartley is in the running for a 2018 seat at Toro Rosso.

Officially, the New Zealander, Le Mans winner and works Porsche driver is simply filling in this weekend for Pierre Gasly, who is in Japan fighting for the Super Formula title.

"There are not so many drivers with a super licence," Marko said in Austin.

"Brendon was always fast as a Red Bull junior, he just didn't withstand the pressure and made mistakes."

Yet Marko, the boss of Red Bull's infamous driver programme, says that Hartley has subsequently become "the fastest Porsche driver" in the Le Mans series.

"We always had contact with him, also through personal sponsorship," he said.

"When it came clear that we needed someone, we wanted someone from our pool. Sebastien Buemi had a contract with Toyota which may not have been a problem, but he also has a contract with Dams [for Formula E].

"But Brendon was released so we decided on him."

For the future, however, nothing is clear. Daniil Kvyat, who was recently benched, confirmed in Austin that he has no idea if he will be called back to race in Mexico, Brazil and Abu Dhabi.

"I can tell you [only] about Austin," Marko said, but he did admit that Red Bull is suffering a shortage of young drivers coming through the ranks, which appears to suggest that Hartley might get a second go at an F1 career.

"That's right," said the Austrian when asked about the talent shortage.

"There is no-one right now in Formula 2 and 3 and GP3, so we took another path and signed four young kart drivers at the end of September."

One of them is Jack Doohan, the 14-year-old son of MotoGP legend Mick.

As for Hartley, Marko said that he performed admirably in Austin practice, comparing well to Kvyat in 'long runs'.

When asked about 2018, Marko said: "We will look at the performance of Kvyat, Hartley and Gasly and then decide about next year."

Toro Rosso are sixth in the team rankings ahead of the United States Grand Prix.