The actress was initially down to portray Maid Marian in Ridley Scott's epic Robin Hood, starring Russell Crowe, but the role was recast to Oscar winner Cate.
"Cate's 10 years older than me, which is more suitable for the script," Sienna told Total Film magazine.
"The script kept changing and evolving to the point where I was no longer appropriate for it."
She also admitted: "If they'd recast with Keira Knightley I'd have been heartbroken but they've gone for something different - and better in my opinion.
"If I had the choice between Cate Blanchett and me I'd go for Cate Blanchett."
Sienna also hinted she thought there was a "vendetta" against her in the media.
"There were several people supposed to be doing the film who no longer are but because it's me it becomes this enormous drama - I think it's a Sienna thing!
"There's just a vendetta against... I don't know."
lol she's kind of crazy, personally i like her but i know most of ontd doesn't, so feel free to take this as you will