Trainer Chris Waller says there's little room for improvement for the world's best horse on turf after she surprised by winning a Randwick barrier trial.

Both Waller and jockey Hugh Bowman appeared content with the fitness of champion Winx who is aiming to join Kingston Town this October as the only horses to win three consecutive Cox Plates.

Winx had never finished top-two in a barrier trial before Tuesday but found herself just off the speed at the turn and improved by two lengths in the straight.

Waller noted the mare was a bit bigger than last preparation but believed the slightly stronger hit-out on Tuesday would have her as good as ever.

"Fitness-wise it's hard to quantify (but) I'd say, after this morning's trial, she's where she normally would be returning," Waller said.

"She's not going any better than she has been in previous preparations but she's certainly not going any worse.

"I wouldn't expect her to (get better) - she's rated the world's best horse on grass. You can't really get much better than that."

Bowman drew parallels with the lead-in to her autumn campaign when he asked her to extend off the bridle during her final hit-out.

While that test was run behind closed doors unlike Tuesday's public trial, the autumn preparation resulted in three Group One victories including the $4 million Queen Elizabeth Stakes.

"I remember the first morning I asked her to stretch herself, she blew in a very similar manner," Bowman said.

"It's pretty much exactly the same this time as last preparation."

The six-year-old, who was last beaten in April 2015, is showing no signs of weariness or a drop in enthusiasm for racing, according to Waller and Bowman.

Winx is aiming to become the only horse besides Kingston Town to win three Cox Plates, Australasia's weight-for-age championship.

Her stable expects the Warwick Stakes to be followed by two or three races before the Moonee Valley showpiece on October 28.

A spring finale at Flemington for the Emirates Stakes has not been ruled out.