The defending Olympic gold medalist is scheduled to represent the United States in Paris in August.
"Yesterday was a tough day, probably the toughest I've had in my 17 years as a professional golfer."
McIlroy's three bogeys in the final four holes opened the door for DeChambeau, who made a brilliant up-and-down on the 18th to seal victory.
Kiwi Ryan Fox carded a six-over 76 to sit 18 shots off the lead.
The golfing legend said it 'may or may not be' his last US Open.
Ryan Fox lies one shot outside the projected cut and eight behind co-leaders Patrick Cantlay and Rory McIlroy.
Ryan Fox is eager to emulate Campbell's 2005 heroics.
Scot Robert MacIntyre held on for his maiden PGA Tour win.
The eight-shot turnaround has him tied for second behind Scot Robert MacIntyre.
