William Buick has confirmed he will ride Thought Worthy in Saturday's Ladbrokes St Leger at Doncaster.
The rider had the choice of partnering the Great Voltigeur winner or Michelangelo for retained trainer John Gosden. but he has opted for the runner he believes has the stronger form.
He said: "I left it late because I wanted to get a feel of just how both horses were at home, but it didn't make things any easier as John Gosden has them both primed to run for their lives.
"Michelangelo breezed on Tuesday and I gave Thought Worthy the same sort of spin on Wednesday and I couldn't split how well they were.
"In the end it was the overall level of form that Thought Worthy has shown which swung my decision his way," he told his At The Races blog.
Buick and Gosden are bidding for their third successive victory in the Doncaster Classic after claiming the last two renewals with Arctic Cosmos and Masked Marvel.
A full-brother to 2007 St Leger winner Lucarno, Thought Worthy finished an 11-length fourth behind Camelot in the Derby earlier in the year before his front-running victory at York.
Buick said: "Although many thought we stole the Great Voltigeur it was really down to his adaptability and courage. I think he's better judged on that performance than the Derby.
"I know Camelot is going to be very hard to get by but both the Clarehaven runners have the ability to bustle him up and I want to be on the one that pushes him hardest and I think that's going to be Thought Worthy.
"He was only just beaten in a bog in the Sandown Classic Trial then came back very gamely to beat Noble Mission at Newmarket after that."
While Buick has opted for Thought Worthy, he also expects Michelangelo to put up a bold show as he sports blinkers for the first time.
He said: "Michelangelo has untapped potential though, and is taking the same route as my first Leger winner, Arctic Cosmos, who was third in the Gordon Stakes at Goodwood.
"Like him, the boss is going to try blinkers for the first time on Saturday and it will definitely help him.
"It's gone down to the wire, I've made what I think is the form call and the right one to give Camelot a fight. I hope I'm right but if not then good luck Michelangelo."
Thought Worthy and Michelangelo are two of nine declared runners, with Frankie Dettori coming in for the ride on the latter.
Dettori partnered Gosden's Shantou to win the St Leger in 1996.
The Clarehaven trainer also saddles big outsider and likely pacemaker Dartford.
Taking star billing, of course, is Aidan O'Brien's Camelot.
The 2000 Guineas and Epsom Derby hero aims to become the first horse since Nijinsky in 1970 to complete the British Classic hat-trick on Town Moor and will be partnered by the trainer's son, Joseph.
Camelot is the sole representative from Ballydoyle after Imperial Monarch and Monday's Galway winner Chamonix were both not declared.
There is another challenger from Ireland in the shape of Tommy Carmody's Ursa Major.
Derby runner-up Main Sequence represents David Lanigan, Encke goes for the Godolphin team under Mickael Barzalona and hugely impressive Melrose Stakes winner Guarantee takes a significant step up in class for William Haggas.
The field is completed by the Henry Cecil-trained Thomas Chippendale, who carries the colours of Yorkshireman, Robert Ogden.