USA: Zenyatta's trainer John Shirreffs has an ominous warning for anyone trying to steal her Breeders' Cup Classic crown - he reckons the undefeated mare may even have improved this year.

"I think she is as good or better than she was last year," said Shirreffs, speaking to reporters in a press conference at Keeneland on Saturday, when he saddled Zenyatta's stablemate Harmonious for a runaway Grade 1 victory in the Queen Elizabeth II Cup.

"She has been at the racetrack in training since she was a two-year-old, so her fitness is good," added the trainer. "The main thing you have to do is keep her mentally happy and wanting to do it."

Zenyatta is vying for favouritism for the $5 million race with top three-year-old Lookin At Lucky at around the 4-1 mark with British bookmakers.

She will arguably face a stronger field this time around as she bids to extend her unbeaten record to 20, and the Classic will be run on dirt at Churchill Downs rather than the synthetic strips on which she has recorded all but two of her victories.

"I think it was a very strong field last year," added Shirreffs. "If you looked at the horses and what they did after the Breeders' Cup, I think it's pretty evident. Twice Over won a couple of Group Is, Rip Van Winkle just won a Group 1."

Despite his confidence, Shirreffs admitted that defeat "would not be the end of the world".

He said: "She has an opportunity to achieve something unattainable but as far as I am concerned, she is just one of the best ever. When she is done, she is going to go off to the pasture and whether she is 19-for-20 or 20-for-20, I think she will be okay."

Despite her unblemished career record, Zenyatta has never been named American horse of the year, having lost out to Rachel Alexandra in 2009 in a controversial decision.

Shirreffs doesn't really want to think about losing the Classic, and possibly missing out on the award for a third time.

"It would be almost too much," he said. "It would be a real slap in the face. I think she deserves it on what she has done on the track and as for the industry as a whole."

Shirreffs was questioned on the possibility that Zenyatta might also be entered for the Ladies' Classic on November 5 as a precaution against a muddy track at Churchill Downs.

"There is one problem with that," he explained. "She leaves November 2. So if she doesn't go there on the 31st you can't cross-enter. You have to be there four days in advance.

"It's the same thing this year as last year. We wanted to run in the Breeders' Cup Classic and that was our choice. That's our choice. If she doesn't leave until November 2, the decision will have been made."