Winx has delivered a performance of sheer dominance never before seen in the Cox Plate to capture the country's weight-for-age championship for a second time.

Saturday's running at Moonee Valley had been shaped as a match race between the reigning champ and the rising challenger Hartnell — in reality it was a one-sided slaughter.

Eight lengths was Winx's winning margin under jockey Hugh Bowman, a record for the race. It was the mare's 13th victory in succession.

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For a fleeting moment the match race looked on as Hartnell and James McDonald made their move at the 800m, but it was covered by Bowman and then some.

Winx ($1.80) surged clear rounding the turn and soon had the home straight to herself, drawing a standing ovation from 26,000 spectators left in awe.

"I think James was quietly confident but when I started to put the pressure on to him, I think he realised the race was over," Bowman said.

"She's just a superior racehorse and it's an honour to be a part of it."

Chris Waller showed his customary emotion, humility and admiration for his horse after the race.

"To get to this stage last year was something different for me, but to get back again with all the build-up is fantastic," the trainer said.

"For her to win like that, it's probably pretty fitting for the horse she is.

"It's like a dream, it's as simple as that."

Winx could hardly have been more dominant in winning her second Cox Plate. ( AAP: Julian Smith )

Winx's performance drew immediate comparisons to past greats of the Australian turf and could see her rated among the world's best for 2016.

She is the 13th horse to win two Cox Plates and the first mare to go back to back since Sunline (1999-2000).

Can Winx join the immortal Kingston Town and win three?

"Getting to the other side of the world would be good but she'll be back next year," Waller said.

"You respect the Australians and they respect her, so that will be right up there at the top of our pecking order."

Hartnell ($5) filled second position after fending off the challenges of Yankee Rose ($13) and French galloper Vadamos ($21).

"Wow. I don't know what to say ... I was going alright too," McDonald said.

"He tried hard, he was brave in defeat, but geez she's unreal."

Hartnell remains favourite for the Melbourne Cup but Godolphin will assess his recovery before committing to a start in the big race.

Trainer David Vandyke, meanwhile, was thrilled with the run of Yankee Rose who is now a short-priced favourite for the Victoria Oaks.

Vadamos raced too keenly according to jockey Mark Zahra but still fought on, while Lucia Valentina failed to figure despite having conditions to suit.

"She just didn't perform around Moonee Valley. She's better than that," jockey Kerrin McEvoy said.

Waller's Grand Cup plan

It was a good day for Waller who earlier prepared a one-two finish in the Moonee Valley Cup led home by Grand Marshal.

The former Sydney Cup winner shortened in Melbourne Cup markets after outstaying his stable-mate Who Shot Thebarman over the 2,500 metres.

"They're in form and heading in the right direction," Waller said of their Melbourne Cup hopes.

"They are genuine two-milers; whether they have the dash to beat the Europeans or Jameka, they'll have a good time trying."

Earlier, The United States returned to winning ways in the Waterford Crystal Mile after illness interrupted his preparation this spring.

The setback might have forced The United States to bypass Winx in the Cox Plate but she may be there for his next target, the Mackinnon Stakes in a fortnight.