The 27-year-old New Zealander admitted as much as he looked ahead on Thursday to his second race with the Red Bull-owned team.
"I think it's clear that the pressure gets higher," he told reporters. "Expectation was quite low with me coming in. I think everyone from the team understood that it was a very tall ask for me in Austin.
"I'm going to do my best to stay as relaxed as I was in Austin. To be honest, as a driver, a lot of the pressure comes from within so I think it's more about me managing that and just focus on what's important."
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Hartley, the first New Zealander in F1 for 33 years, was initially called up as a replacement for Pierre Gasly while the French rookie tried to win the Super Formula series in Japan. He had last driven a Formula One car in 2010.
Spaniard Carlos Sainz had also departed for Renault, opening up a seat for Russian Daniil Kvyat to return after being dropped for Gasly in Malaysia and Japan.
Red Bull decided after the U.S. Grand Prix that Hartley and Gasly would continue for the rest of the season but Toro Rosso's 2018 lineup remains undecided even if the current pair are favourites.
Hartley, who finished 13th in Austin, said he had been told only on Monday that he was continuing.
Mexico has particular resonance to his compatriots as the circuit where the late Denny Hulme won the championship 50 years ago.
"I'm really looking forward to it," he said.
"I learned a lot in Austin. It was a pretty steep learning curve ... looking forward to putting a lot of the stuff I learned into practice. I'm feeling pretty confident about it.
"I think this weekend I'll have the capacity to play a bit more with the car and understand a bit more. There was a lot to focus on in Austin so they kept it pretty simple for me."
A Le Mans winner and world endurance champion with Porsche, Hartley said he had plenty of experience of handling pressure with a big team geared up for one race above all others.
"These cars are very physical to drive and they should be. I was a bit stiff after the race, the position compared to LMP1 (the top class of endurance racing) is very different so you activate a few different muscles.
"I was pretty happy how I dealt with it but I'm not going to lie, I could feel my neck and a few other parts of the body. That's normal."
(Editing by Greg Stutchbury)
