Hinchcliffe and Rossi arrived at Mount Panorama as Great Race dark horses after accepting a wildcard entry with Holden heavyweight Walkinshaw Andretti United for Sunday's premier enduro.

However, they received a quick reality check when they finished at the back of the 26-strong field after Thursday's first day of practice.

Rossi recorded the best lap time of the pair - two minutes, 07.22 seconds - in the third and final practice stint.

But it was still almost three and a half seconds slower than pace setter Scott McLaughlin, albeit after the Ford gun set a new lap record of 2:03.77.

Either way, the IndyCar stars admitted they faced a crash course in Supercars if they were to threaten for a podium finish on Bathurst debut this weekend.

"It's a bit of a baptism of fire for us here," said Hinchcliffe.

"Alex and I were joking earlier it's kind of like drinking out of a fire hose.

"We have very little car knowledge, zero track knowledge and obviously this is one of the most daunting, challenging race tracks in the world.

"But man it's beautiful. All the hype, it's everything everybody said it was and then some."

Hinchcliffe last got behind the wheel of a Supercar back in 2012 on the Gold Coast while Rossi has never raced in the category, let alone seen the notorious Mount Panorama track.

And alarm bells seemed to be ringing for the IndyCar stars after it emerged Rossi ran off track at Supercars testing at Winton last week, cutting short their three-day session by 24 hours.

Yet the pair were talked up as outside chances of a Bathurst boilover, with Ford star Chaz Mostert backing them to cause an upset in Sunday's 161-lap classic.

Certainly no one can argue with their resume.

Rossi first came to prominence by claiming the famous Indianapolis 500 in 2016 while Hinchcliffe has such a high IndyCar profile he has earned a spot in US TV's Dancing with the Stars.

However the pair admitted they felt like rookies after a humbling practice day.

"It was a lot to learn," said Rossi.

"What we keep reminding ourselves is not only is this a new track for us, but it's a car that we're not completely comfortable with either.

"We're trying to figure out all of those things at once, around a place that doesn't have a margin for error.

"We'll just have to keep chipping away at it tomorrow."

Hinchcliffe admitted they would be doing some serious homework before the final two practice sessions ahead of Friday afternoon's qualifying session.

"I think both us of will have a lot to sleep on tonight," he said.

Australian Associated Press