Roger Federer's rapid decline continued after defeat in straight sets to Tommy Robredo at the US Open.
The 17-time grand slam champion was seeded seventh at Flushing Meadows, his lowest ranking for 11 years and will not compete in a major final during a calendar year for the first time in a decade.
Robredo beat the Swiss 7-6 6-3 6-4 in New York to equal his best performance at a grand slam where he now faces fellow-countryman Rafael Nadal who defeated German Philipp Kohlschreiber in four sets.
But the focus remains on Federer who has slumped to seventh in the world following his round two exit at Wimbledon and the former world No.1 couldn't pinpoint the problem with his game.
"I kind of feel like I beat myself, you know, 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. I kind of self destructed, which is very disappointing, especially on a quicker court.
"Your serve helps you out. You're going to make the difference somewhere. I just couldn't do it. It was a frustrating performance today.
"If I'm playing like this, I'm not going to beat Rafa, or Kohlschreiber, for that matter. So, 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. Not thinking too far ahead of myself, especially the level I was playing at.
"I've definitely got to go back to work and come back stronger, you know, get rid of this loss now as quick as I can, forget about it, because that's not how I want to play from here on," the 32 year old said.
"I want to play better. I know I can. I showed it in the last few weeks, that there is that level. So today was pretty frustrating."