Star apprentice Ben Allen has won the opening race on Blue Sapphire Stakes day wearing the colours he hopes to carry to victory in the $5 million Caulfield Cup.

Allen rode Azuro to victory on Wednesday and hoped it was a sign of things to come as he prepares to partner Night's Watch is Saturday's Caulfield Cup over the same course.

Like Night's Watch, Azuro is trained by Darren Weir and races in the colours of syndicator OTI Racing.

"I'm hoping we can get a repeat on Saturday," Allen said.

"That would be lovely."

Night's Watch is one of three Weir-trained runners in the Caulfield Cup along with favourite Kings Will Dream and Gallic Chieftain.

Despite being only 19 years old, Allen is no stranger to riding in the rich handicap.

Saturday will be his third ride in the Caulfield Cup having finished 14th on Pemberley in 2016 and fourth on Lord Fandango last year - who were also for OTI Racing.