Sir Mark Prescott fears Marsha will struggle to confirm her superiority over Lady Aurelia and give him a winner with his first ever runner at next month's Breeders' Cup at Del Mar.
The long-serving Newmarket handler will break new ground when the four-year-old renews her rivalry with Wesley Ward's dual Group One winner in the Turf Sprint on November 4.
Although the four-year-old proved no match for Lady Aurelia in the King's Stand Stakes at Royal Ascot, the daughter of Acclamation turned the tables with a narrow victory in the Nunthorpe Stakes at York in August.
But despite the Elite Racing Club-owned filly, who completed her preparations for the Grade One prize with a spin around Chelmsford City on Wednesday, gaining the upper hand last time out, Prescott believes the tight nature of the California track is not in her favour.
He said: "I've been to America, but only to the sales, so it will be a completely new experience. I've sent horses over there, but I've always managed to avoid going myself.
"We've been to Chelmsford today (Wednesday) to go around a left-hand bend. She went OK, but she didn't go around there like a greyhound.
"She was going so quick when she got to the turn. I would have liked her to have gone around tighter. It wasn't by any means bad, but it is not going to be to her advantage.
"It is her last race and the Americans were unbelievably kind and generous in getting her to go over.
"She goes to the sales at the end of November. We have really got nothing to lose and everything to gain. They felt they wanted her to be there to make the race more interesting."
He added: "She has been great and has got better and better as she has gone.
"She has experience of going left-handed. She has won at Catterick and at Dundalk, but they were when she wasn't going nearly as fast. Now she goes quicker I'm not sure how she will go around the turn.
"The fast pace and fast ground she will love. Lady Aurelia will just whizz around those turns in her own back yard. I can't imagine from everyone's description of Del Mar that there will be enough time for Marsha to make up the ground.
"At York everything was in her favour and poor Battaash got in a right stew and Mr (Charlie) Hills rectified that next time and we were probably lucky to get up on the line at York. I am not really expecting that much luck this time."
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