Brendon Hartley has admitted that getting up to speed straight away in Austin will be difficult.

Toro Rosso owner Red Bull has signed the New Zealander and Le Mans winner to replace Pierre Gasly this weekend, as the Frenchman is racing in Japan.

Hartley, a former Red Bull reserve driver who has never raced in F1, admits that he is studying hard.

"I was given instructions for the car which is 50 pages," he told Newstalk ZB.

"I will have four hours of practice before qualifying and I'd like to think it's enough to get the speed I need."

The 27-year-old thinks that he was selected by Red Bull because of his success in the top Le Mans category LMP1, where he is a top driver for Porsche.

"If we talk about laptime, the difference is not so great - around seven to eight seconds," he said. "That is not so much, especially in the race.

"I drive very fast cars, so I think Red Bull invited me partly for this reason but also because WEC cars are very technically complex. We use hybrid technologies, so in this regard it is similar to Formula 1.

"But it's a big step for me, because six or seven years have passed since I drove open wheel cars. But I try to look at things simply as well."

"It's a racing car, it has four wheels, and for a driver to go fast you have all the same physical laws. So in principle it's similar."

There is speculation that Hartley's appearance in Austin will not actually be a one off, as he could be asked to return for the final three races of 2017 in the place of Russian Daniil Kvyat.