The decision to keep Hartnell in Sydney rather than climb the ladder to the Group One Australian Cup has paid dividends in a lesser race at Rosehill.

One of a group of middle distance and staying horses purpose bought by Godolphin in Europe for Australian racing, Hartnell showed his class to win Saturday's Group Three Sky High Stakes (2000m).

Trainer John O'Shea admits he is still learning about Hartnell, second to stablemate Contributer in the Chipping Norton Stakes and a two-length winner over Opinion in the Sky High.

"I kept him here because I am still learning about him," O'Shea said.

"The softer ground in Sydney I thought would suit him and I don't want to overtax him.

"He and Contributer came to me in great order from Mark Johnston and I just want to make sure we do the same."

Hartnell was the $1.45 favourite with Opinion at $18.

Opinion's Chris Waller-trained stablemate Hawkspur ($31) ran his best race for some time to run a head third.

With Waller in Melbourne to oversee Brazen Beau's Newmarket Handicap win, his racing manager Liam Prior said the Sky High placings were pleasing.

"They are on track for the longer races over the carnival," he said.

O'Shea said he was uncertain where Hartnell would head next.

"I had no idea whether he could back up in two weeks and he did," he said.

"I don't if he can again.

"The idea is to get his rating up so he can get into the Melbourne Cup. That is his aim."

Hartnell's win was James McDonald's first for Godolphin since he was signed as a retained rider two days ago.