"The ball from Roger, it comes like fire, no? All the time it's so, so fast," BBC Sport quotes him as saying. "You have to be trying so hard with your legs to go down and try to put the ball in," he said.
"If I wanted to beat him, I had to put the match very tough, to play all the rallies as long as possible, and try to let him play on the backhand, no?
"For me, Roger for the moment is the best player of all times. And to beat him in a huge stadium like the US Open and in a Grand Slam, a match of five sets, it's like a dream. I am so, so happy. It was a great day."
This is the first time that Spaniard Robredo has reached the last eight at the US Open.