"It was very, very tough conditions today but I sweat too much," BBC Sport quotes him as saying. "The opponent was great, he had good chances in the first set and I feel lucky to be through. I fought a lot.
"[Robredo's] playing great, I'm happy for him. He has similar circumstances to me, he came back after a tough injury. It's not easy.
"Today, winning against Roger in the US Open is just amazing and he will come into my match with big confidence."
The win saw Nadal extend his winning streak on hard courts to 19 matches.