A three-year-old running in a Group One race over two miles is becoming a novelty in Australian racing.

In Darley's case, it is unheard of but what Australian Derby runner-up Tupac Amaru doesn't know about the challenge ahead won't hurt him.

At least that will be trainer Peter Snowden's approach to the $1 million Sydney Cup at Randwick on Saturday.

"Weights and measures say he can't possibly win but I haven't told him that, he doesn't know it," Snowden said.

"I think he's going to run well."

The last three-year-old to win the Sydney Cup was Tie The Knot in 1998 when he carried 50.5kg.

Tupac Amaru will carry 54kg, just 4kg less than Irish Group One winner and topweight Voleuse De Coeurs.

Snowden is the first to admit it is a genuine task for a young, inexperienced horse.

But Darley identified the colt as a dyed-in-the-wool stayer early in his career and they are keen get a guide on his spring Cups potential.

"To me it's about testing the horse. He's ready to run two miles in my mind," Snowden said.

"He's a very dour stayer, a very tough stayer. At the end of the day, unless you have a go, you don't know.

"It's also food for thought for us later next preparation. If we know he's a genuine stayer we can look at those genuine distance races knowing we've got a good chance."

Gai Waterhouse's in-form stayer The Offer continues to be the best supported Cup runner as he holds down favouritism at $2.80.

Randwick trainer John Thompson likes the chances of Tremec figuring in the finish after the no-nonsense stayer has chased home The Offer at his past two starts.

A $16 chance, Tremec didn't come up in the spring but the five-year-old is in a similar patch of form to the one that carried him to successive staying wins during last year's autumn carnival.

"The favourite has beaten him convincingly at their past two starts but he's coming right at the right time and that's the main thing," Thompson said.

"He's there, he's fit and he's well and I know he will run the distance.

"Each run has been a bit better. He pulls up well and he backs up. I can't get him any better."

Tremec had no answer to The Offer's finish in the Chairman's Handicap last week and meets him on worse weight terms on Saturday but Thompson is banking on an improved racing surface to work in his horse's favour.