The Spaniard was thrashed by his compatriot and world number two with a 0-6 2-6 2-6 defeat at Flushing Meadows in the last eight on Wednesday.
Robredo feels that his slow start to the match allowed Nadal to run away with the clash.
"I started a little bit tight and he was up very quick," Robredo told reporters. "Then there was nothing else to do. He was too good."
Nadal will now meet Richard Gasquet in the last four of the competition.