Some professionals never make it to a Major 1/4 final in their entire careers.
Fed loses a couple of times in the quarters, and behold, the end of the world is upon us.
He's not moving quite as well as before, and his motivation/intensity seems to be lacking somewhat.
Also, he doesn't convert opportunities (break points) as well as he was doing before.
2 or 3 important points that one doesn't win in a match can be costly (i.e. make the difference between winning and losing).