Hartley, who's been part of Porsche's World Endurance Championship line-up since 2014, made back-to-back F1 appearances behind the wheel of a Toro Rosso STR12 in Austin and Mexico in late October.

He returned to his full-time Porsche role last weekend in Shanghai and sealed his second WEC title – but has now admitted he did not find it easy to readjust to the 919 Hybrid LMP1 car.

"It was actually surprisingly tricky to get back in the car," Hartley recalled. "I thought I would feel right at home, [it's] been home for the last few years.

"Took a few laps to feel comfortable again - having a roof over your head, the seat position is different."

Despite his LMP1 experience, Hartley says his first taste of the 2017-spec F1 car was "mindblowing" and "a shock to the system".

Asked what surprised him about grand prix racing, the New Zealand racer said: "I guess, Austin, the big shock was high speed, just in terms of driving.

"They are amazing, these cars, especially in the first sector. It was mindblowing, so it took some getting used to. You always want more grip and more power, but it was a bit of a shock to the system.

"No other real surprises, I knew it was going to be a challenge, I knew the level of driver/teams was the highest you can find - I guess there were no big surprises and I kind of expected that."

Hartley, who will see out the season with Toro Rosso and is widely expected to land a full-time drive for 2018, reckoned his experience of driving for a major manufacturer in WEC prepared him for F1's complexities.

"From working with Porsche the last years, it made my transition quite smooth," Hartley said. "It is a similar number of people involved, the structure, the pressure - driving at Le Mans with Porsche is a big amount of pressure.

"All those things I have learned to deal with, but there are a lot of other things to learn. I am aware there are still a lot of things to learn."

Amid a run of seven consecutive race weekends across F1 and WEC, the Kiwi insisted he has not yet had the time to reflect on the milestones he's reached this year.

"I've tried not to overthink it," he said. "Even winning the [WEC] championship hasn't sunk in, I haven't had time to reflect, going from one race to another.

"It won't be until the end of year when I am home that I can think about it all. I am aware that the F1 debut, Le Mans victory and WEC championship are all amazing things but at the moment I am trying to take one step at a time, so I stay focused and catch up on as much sleep as I can."

Additional reporting by Jonathan Noble