Ronaldo labels Ashley Cole his toughest opponent

Cristiano Ronaldo has never gone up against an opponent tougher than Ashley Cole.

Cole's tenacity was one of the primary reasons Ronaldo named him as his most frustrating adversary, as the pair clashed on several occasions when the left-back played for Arsenal and Chelsea while the Portuguese star was at Manchester United.

Although Ronaldo has been challenged by some of football's top defenders in Spain and other competitions since he left United for Real Madrid in 2009, the three-time player of the year winner continues to hold Cole - who currently plays in Major League Soccer with the LA Galaxy - in high regard.

"Over the years I had some great battles with Ashley Cole, he does not give you a second to breathe," Ronaldo admitted to Diego Jokas of Coach magazine. "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."

Ronaldo went on to talk about another important figure during his time in England when he cited former United manager Sir Alex Ferguson as the manager who's had the biggest impact on his career.

"I do see Sir Alex Ferguson as a father figure," he said. "All my coaches have been important, but when I arrived at Real Madrid I was a man, when I arrived at Manchester United I was a boy. All of a sudden you are at one of the biggest clubs in the world, and at that time I needed somebody like Sir Alex."