However, he is closing the gap each time, getting to within 1 lengths of his rival in the Lightning Stakes last start.
The Royal Ascot 1200m course has a gradual incline to the finish and is a stamina test for sprinters.
Black Caviar and Hay List have already won group one races over 1400m and appear well suited to the tough English track conditions.
Sepoy is yet to race beyond 1200m but any horse who can succeed in the pressure-cooker environment of a Blue Diamond Stakes and then Golden Slipper should cope with the testing track.
So what are the chances of this Royal Ascot showdown?
Black Caviar's autumn race programme has been tailored to ensure the mare is at her peak for the Diamond Jubilee.
Sepoy is also being geared for the sprint feature race via the Golden Shaheen in Dubai on World Cup night at the end of this month.
Hay List, who is now a starter in next week's Newmarket Handicap, could also line up at Royal Ascot if he wins the TJ Smith Stakes at Royal Randwick and the KrisFlyer International Sprint in Singapore in early May.
Black Caviar, Hay List and Sepoy are the headline act of the Aussie invasion to England. They are likely to be joined by Foxwedge and Ortensia.
Hay List's owners, the Davenport family, have had a change of heart and yesterday decided to run their powerhouse sprinter in the $1 million Newmarket Handicap at Flemington on March 10.
"It's a big weight but we wouldn't be running him if we didn't think he could win," part-owner Peter Davenport said. "He's been trained for the Newmarket. Rather than run and hide, we're going to run.
"Hay List is good enough to carry 58.5kg and win a Newmarket but can he give that racmuch weight to those horses?"
Glyn Schofield, Hay List's regular rider, had already taken the mount on last year's Cox Plate winner Pinker Pinker in the Chipping Norton Stakes at Warwick Farm next week but was granted a release so he could ride at Flemington.
Trainer Greg Eurell secured Blake Shinn to ride Pinker Pinker in Sydney's first group one race of the autumn carnival.
