Roger Varian's grey has been the horse for money all week in the final Classic of the Flat season, partly because he has proven his effectiveness with cut in the ground.
Defoe began the season with a paltry rating of 88 but he has taken a huge step forward with each run.
His progression began with a win in the informative London Gold Cup at Newbury and he defied a 10lb rise to win at York next time out.
Upped to Listed class at Hamilton, he beat a subsequent Group Three winner in Frankuus before taking his form to a new level on his most recent outing.
Up against older horses in the Geoffrey Freer Stakes he oozed class, beating Wall Of Fire cosily, but that form still leaves him shy of Group One company.
"He's in great form, I'm looking forward to riding him," said Atzeni.Defoe is the only horse in the William Hill St Leger field on Saturday who is unbeaten this season (PA)
"He needs to improve again, but I think he has. The ground should suit him, he stays well and we'd be very hopeful."
Defoe is currently 9-2 third favourite with the sponsors.