The confidence of Jim Cassidy and the training brilliance of Chris Waller combined to confirm the outstanding colt Zoustar as Sydney's best sprinter of his age at Randwick on Saturday.

And the next time he's seen, Waller and Cassidy expect him to prove himself Australia's best.

Zoustar defied a couple of notable obstacles to win the Group Two Roman Consul Sakes (1200m), earning himself a run in the Group One Coolmore Stud Stakes over the same trip at Flemington on Derby Day.

"He had to beat them like he did to go to Melbourne," Cassidy said.

"Now he's done it, he'll do it again. He's a superstar."

Waller performed the sort of training feat that has made him Australia's best by bringing Zoustar back from a Group One win at 1400m to Saturday's shorter race - and Cassidy did the rest.

After slightly missing the start the pair idled along in second last spot until turning for home.

At the 400m they had passed half the field, at the 300m Cassidy had a cheeky look around and joined the lead and at the finish they were 4-1/4 lengths clear of the runner-up Barbed ($7).

"I knew nothing was going to go with us," he said.

"But I had a look to make sure. I was right.

"He was never out of second gear."

Cassidy had the opportunity to ride Hawkspur for Waller in the Group One Turnbull Stakes at Flemington on Saturday, but chose instead to stay at Randwick to handle Zoustar.

Waller said the show of confidence was welcome, but he never doubted his colt.

"I just told Jimmy to keep it simple," he said.

The trainer said he had considered a Caulfield Guineas preparation for Zoustar, but decided instead to stick to sprinting until at least next year.

"The autumn's there for him, there's no need to try anything fancy."