He started the weekend tied for 29th place, which means he came from farther back than any other champion since 1934.
This year, the Open returns to Merion for the 12th straight decade and it will be the 18th USGA Championship to be played on the famed course dating to the 1916 U.S. Amateur.
The field includes the top players in the world, including world No. 1 and three-time winner Tiger Woods. Woods is having a dominant season with four victories in eight events this year, but is coming off a disappointing showing at the Memorial two weeks ago, where he did not shoot a single round in the 60s.
He also has not won a major since the 2008 U.S. Open.
Woods, placed in one of the marquee groups over the first two days of the tournament, will be playing with 2011 U.S. Open champion Rory McIlroy and Masters winner Adam Scott, who are ranked second and third in the world, respectively.
Simpson, meanwhile, will seek to become the first person since Curtis Strange in 1988-89 to win back-to-back U.S. Opens.
Other previous winners in the field are Graeme McDowell, Lucas Glover, Angel Cabrera, Geoff Ogilvy, Michael Campbell, Jim Furyk and Ernie Els.
ESPN will have early and late coverage over the first two days, while NBC will have afternoon coverage on Thursday and Friday before taking over full time for the weekend.
The PGA Tour heads to Connecticut next week for the Travelers Championship, where Marc Leishman fired an 8-under 62 in the final round last year to capture his first career victory on the PGA Tour.
NAJETI HOTELS ET GOLFS OPEN, Aa St. Omer Golf Club, St. Omer, France - Darren Fichardt tripped to a 2-over 73 during the final round at this event last year, but it was still good enough for him to capture his third career victory on the European Tour.
Fichardt will not be in the field this week to defend his title.
Dawie Van Der Walt is the lone winner this season who is participating in the event, which consists of a relatively small field.
There will be no television coverage this week.
The European Tour will move to Germany next week for the BMW International Open, where Danny Willett collected his first European Tour title last year by defeating Marcus Fraser on the fourth playoff hole.
AIR CAPITAL CLASSIC, Crestview Country Club, Wichita, Kansas - Casey Wittenberg came from behind with a 5-under 66 in the final round to capture his second title of the season at this event last year.
Wittenberg will not return to defend his title this year, but three former champions -- Steve Wheatcroft, Jeff Klauk and Paul Claxton -- are in the field.
There will also be no television coverage of the Web.com Tour this week.
The tour heads to North Carolina next week for the Rex Hospital Open, where James Hahn birdied the second playoff hole to beat Scott Parel and earn his first win on the Web.com Tour last year.