Roger James doesn't know if Rondinella will sink or swim when she steps up to two miles for the first time in the Sydney Cup at Randwick, but the horse he considers his best since multiple Group One winner Silent Achiever has given him every reason to find out.

The experienced New Zealand horseman, who formed a training partnership with Robert Wellwood at the start of the season, did not have the Sydney Cup high on his agenda when he travelled Rondinella across the Tasman a few weeks ago for a hit-and-run raid on the Tancred Stakes at Rosehill.

But when the four-year-old bounced out of her game third to weight-for-age star Avilius on an unsuitable heavy track, her connections decided to stick around for The Championships.

"She thrived during that week so we decided to keep going," James said.

"Look I made a big statement when she won her maiden, that I thought she was the best I'd had since Silent Achiever, and I've got no reason to regret making that statement."

The winner of four of her 12 starts, Rondinella will be in uncharted territory when she is stretched to 3200 metres in Saturday's $2 million Sydney Cup at Randwick.

With an inconclusive pedigree and no form at the distance to go off, James admits he is in the dark when it comes to assessing her suitability for such a test of stamina, especially against the likes of Godolphin's international raider Dubhe.

Rondinella's sire Ocean Park was a middle-distance horse while her dam, Our Valpolicella, did not race beyond 2100m after she was injured in the lead-up to the 2008 Queensland Oaks.

"It's a big query," James said.

"She did relax beautifully in the Tancred the other day, as good as I've seen her relax.

"I think her best distance will probably be a bit shorter than this but the weight is a factor, it might help her get a bit further having no weight."

Rondinella will carry 52kg in the Sydney Cup after stewards gave Sam Clipperton permission to ride her a kilo over her nominated handicap.

She was an $18 chance on Thursday with betting headed by Dubhe, the $3.40 favourite to give Godolphin's English-based trainer Charles Appleby a second Sydney Cup after he won his first two years ago with Polarisation.

Lloyd Williams will be represented by $8.50 second pick Midterm who will bid to join Gallante (2016), Mourayan (2013) and Gallic (2007) as the leviathan owner's recent winners of the race.