The sole filly in this year’s $1.5 million Group 1 AJC Australian Derby (2400m), the Danny O’Brien-trained Shamrocker, has been one of the best backed runners to claim an upset over glamour Kiwi colt Jimmy Choux at Randwick on Saturday.

While last-start Rosehill Guineas winner Jimmy Choux remains the favourite to win the 2011 AJC Derby after the Thorn Park colt came up trumps with barrier one, odds on second elect Shamrocker have shortened since final field markets opened yesterday.

Shamrocker has firmed in from $7.50 to $6.50 while Jimmy Choux has drifted slightly out from $2.10 to $2.15.

Runner-up to Brazilian Oaks in the Group 1 Crown Oaks (2500m) during the spring, O’reilly filly Shamrocker resumed with a first-up fourth to Southern Speed in the Group 3 The Mittys Vanity (1400m) on February 19 before she claimed a shock win in the Group 1 Australian Guineas (1600m) at Flemington on March 5.

Last start Shamrocker ran a credible third beaten one and a half lengths by Jimmy Choux in the Group 1 Rosehill Guineas (2000m) on March 26.

On Saturday she has drawn barrier eight, the same gate she jumped from in the Rosehill Guineas, but will re-partner with her Australian Guineas winning jockey Glen Boss.

Boss remains confident that Shamrocker can return to form over the extra distance on Saturday and turn the tables on Jimmy Choux.

“Jimmy Choux is obviously the one to beat,” Boss said.

“He is very good as we saw in the Guineas, but Shamrocker will get the trip and can test him with a clear run.”

Jimmy Choux, however, is also ideally suited to the AJC Derby distance having won the Group 1 New Zealand Derby (2400m) on March 5.

History is also against Shamrocker being a filly.

This year marks the 150^th running of the Australian Derby and in the previous 149 editions of the autumn feature only eight fillies have claimed victory.

Luxbet’s chief bookmaker Paul Vincent is also against Shamrocker for the Derby win.

“I’ll oppose Shamrocker,” Vincent told Races.com.au.

“I believe only champion fillies can win the Derby and she’s not that.”

The history doesn’t support Jimmy Choux either, however, if late money comes firming him into odds-on territory.

The last horse to start odds-on in the AJC Derby was Carnegie Express who was defeated by Don Eduardo as the $1.75 favourite in 2002.

Only one other AJC Derby hopeful has firmed in the markets with Domesky in from $41 to $31.

The Michael Kent-trained Domesday gelding won the Group 2 Alister Clark Stakes (2040m) at Moonee Valley on March 18 before failing to come within seven lengths of Jimmy Choux finishing 10^th in the Rosehill Guineas after jumping out wide from gate 11.

Domesky has drawn better with barrier three for Saturday, which should see them come closer.

The 2011 David Jones 150^th AJC Australian Derby will run as Race 7 at Randwick on Saturday jumping at 3:40pm (AEST).