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The five-time Ballon d'Or winner came up against some of the best defenders in world football during his seven year spell with Manchester United.
John Terry from Chelsea was at the peak of his powers, as were the likes of Nemanja Vidic and Rio Ferdinand.
But in an interview with El Chiringuito TV back in 2018, Ronaldo revealed that three-time Premier League winner Cole was a tricky customer.
It wasn't the first time the 35-year-old had expressed his admiration for Ashley Cole's defending ability either.
Speaking to Coach Magazine back in 2016, he said: "Over the years I had some great battles with Ashley Cole, he does not give you a second to breathe.
"He was such a tenacious player when he was at his peak, quick, tough in the tackle. You knew it would never be an easy game."
It's certainly high praise from the Juventus forward and looking back, the former Arsenal full-back contained Ronaldo on a number of occasions.
And it's easy to see exactly why Real Madrid's all time top scorer thinks the former Arsenal man was his toughest opponent.
The pair met several times down the years, perhaps most notably at international level, and this short highlights video shows that Cole definitely had some great moments against his sparring partner.
While Ronaldo feels Cole gave him the most trouble, the feeling isn't mutual.
"It has to be Messi," the 40-year-old said.
"People talk about Ronaldo more because I played against him more. But I think to mark Messi, on his day... he was just too good for me.
"But, again, he's never scored [against me]."
Do you think Ashley Cole is one of football's greatest ever full-backs?
Let us know in the comments.
Words by Jack Kenmare and Ryan Sidle
Featured Image Credit: PA
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