Ben Melham hopes he hasn't pulled the wrong rein in selecting I Am Immortal ahead of Brooklyn Hustle in the Blue Diamond Stakes at Caulfield.

The jockey rode both two-year-olds in divisions of the Blue Diamond Preludes on February 9 and was left with the choice of two leading chances.

While Brooklyn Hustle finished fifth in the Prelude, Melham was impressed with her performance but he had also ridden I Am Immortal to victories in the Blue Diamond Preview and Prelude.

"It wasn't easy. They're both very good two-year-olds and it wouldn't shock me to see either one win," Melham said.

"I won the two lead-up races on the colt (I Am Immortal) so I'd be disappointed if he won on Saturday and I wasn't on him."

Compounding the problem is his close association to both trainers.

Anthony Freedman prepares I Am Immortal with Jason Warren the trainer of Brooklyn Hustle.

"I was apprenticed to Anthony and I've ridden a lot for Jason throughout my career and that made it even more difficult," Melham said.

"I spoke to Anthony Mithen (managing owner of Brooklyn Hustle) to see if it was going to be a problem if I didn't ride the filly and they were happy to put Beau Mertens back on.

"All's well that ends well, so long as she doesn't beat me on Saturday."

Melham sees the first half of Saturday's race as crucial for I Am Immortal.

He believes if he can relax in the first 600m, they won't catch him.

"But it's a fine line. You have to go with him a little bit but you don't want him running too quick so there's no horse left at the finish," Melham said.

"He's such a competitive colt, if something comes at him during the race he wants to race.

"It's a good quality so we want to save that quality for the finish, not in the first 600 metres.

"He's a very good colt and Anthony's a very good target race trainer.

"I think his two wins along the way have been a bonus and the Diamond is the race he's been set for."

I Am Immortal and Brooklyn Hustle share the fourth line of Blue Diamond betting at $9.50 with Loving Gaby retaining favouritism.