Brendon Hartley found the high-speed grip offered by the 2017-spec Formula 1 cars "an eye-opener" after enjoying his first run-out with Toro Rosso ahead of his grand prix debut on Sunday in the United States Grand Prix.

Porsche LMP1 driver and Le Mans winner Hartley was drafted in by Toro Rosso to replace Pierre Gasly for the US GP race weekend, with Friday's practice in Austin marking his first run in an F1 car since 2012.

The New Zealander enjoyed a solid first day behind the wheel of the STR12 car, but noted a couple of big challenges between F1's new cars and the LMP1 machinery he is used to.

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"A steep learning curve today! It all went pretty smoothly and I kept the car on track without making too many mistakes, so I’m quite happy," Hartley said.

"I didn’t really know what to expect from today because I just had so much to learn. I think I made quite a big progression throughout the day.

"The biggest difference from what I’m used to is the high-speed grip, it’s incredible here in Formula 1… it was quite an eye-opener! Another challenge are the tyres, which are also quite different to what I’m used to.

"On the other hand, the long-run looks quite positive and I did a good job managing the tyres there – the biggest thing I need to work on now is the new tyre pace, and I’ll get another crack at it tomorrow morning before qualifying.

"All in all, I’d say it’s all coming together. We’ll now work hard and go through plenty of data tonight and hopefully I’ll make another step forward tomorrow.”