The 17-times grand slam champion was eliminated 7-6(3) 6-3 6-4 in less than two-and-a-half hours to end hopes of a blockbuster showdown with great rival Rafa Nadal in the quarter-finals. (Agencies)
The Swiss's erratic display included a most Federer-like 43 unforced errors and left 14 of 16 break points going begging at Louis Armstrong Stadium.
"Confidence takes care of all the things you don't usually think about," Federer told reporters, his stinging defeat reminiscent of his shock second-round exit at Wimbledon two months before.
"Maybe my consistency is just not quite there yet. Maybe on a daily basis, set-by-set or point-by-point basis, maybe that's something that has been difficult for me,” he said.
"Maybe that was one of the reasons why I lost today, playing up and down. I kind of self-destructed, which is very disappointing," Federer said.
Another bout of foul weather took Federer away from his favoured center court at Arthur Ashe Stadium, relegating the former world number one to the secondary show-court, where he had not appeared since 2006.
Although filled to capacity, Federer gave the Louis Armstrong Stadium crowd precious few reasons to roar as he struggled against an opponent he had beaten in all 10 of their previous meetings.
"I kind of feel like I beat myself, without taking any credit away from Tommy," Federer said.
"Clearly he was making sure he was making many balls. It was up to me to make the difference and I couldn't. It was a frustrating performance today," he added.
Federer appeared impatient on most of his break point chances, miscuing a stream of groundstrokes to let Robredo off the hook.
It was his earliest exit from the year's last grand slam since a fourth round loss in 2003.
"I think I explained it enough. It just ended up being a bad combination of many things today. I've definitely got to go back to work and come back stronger," Federer said.
"Get rid of this loss as quick as I can, forget about it, because that's not how I want to play from here on. I want to play better. I know I can. I showed it the last few weeks, that there is that level," he said.
Federer said the much-hyped clash with Nadal was the last thing on his mind when he took to the court and defended his usual laidback appearance as the game slipped from his fingers.
"If I'm playing like this, I'm not going to beat Rafa, or Kohlschreiber for that matter. For me, I didn't think of that. I've been too often in this situation. I was fighting with other things in my match today," he said.
The 17-times grand slam champion was eliminated 7-6(3) 6-3 6-4 in less than two-and-a-half hours to end hopes of a blockbuster showdown with great rival Rafa Nadal in the quarter-finals.