At age 52, and for the third year in a row, Couples put himself into title contention going into the weekend and gave himself a chance to become the oldest major championship winner in golf history.
He shot the day's low round in cool, breezy weather -- a 5-under 67 that began with a bogey and ended with a dramatic par save after his tee shot at the par-4 18th hit a tree.
Couples was tied for the lead at 5-under 139 with Jason Dufner, who last summer had the PGA Championship in his grasp before falling in a playoff to Keegan Bradley.
"It's very shocking and it was a great day," Couples said. "I've said it for 28 years, this is my favorite golf tournament in the world. This was an incredible round, a very, very good round."
Couples' climb to the top highlighted a day in which three-time champ Phil Mickelson jumped back into contention and four-time Masters winner Tiger Woods did not. Mickelson's 68 left him at 142 and made him one of 18 players within three shots of the lead heading into the third round.
Woods did not come close to resembling the player who won two weeks ago in Florida to end a tour drought that was in its third year. He wound up with a 75, within a shot of his worst score at the Augusta National Golf Club as a professional, and was in a tie for 45th place at 147.
Couples and Dufner found themselves one stroke in front of a group of seasoned international stars -- Sergio Garcia, former British Open winner Louis Oosthuizen, long-hitting Bubba Watson and the players currently ranked second and third in the world, Rory McIlroy and Lee Westwood.
The late Julius Boros was 48 when he won the 1968 PGA Championship in a duel over Arnold Palmer, and although there have been many close calls since then, no one older has claimed one of the major titles. Tom Watson, at age 59, lost in a playoff at the British Open to Stewart Cink three years ago.
Couples was a shot out of the lead at the 36-hole mark of this event last year before shooting 72-73 on the weekend and finishing in a tie for 15th.