He was on the wrong end of a three-shot swing as Chappell birdied the same hole from 10 feet.
U.S. Open champion Johnson steadied the ship to birdie the par-five 18th for a one-under-par 69, his eighth consecutive round in the 60s but one that left him ruing what might have been.
"This course is very difficult off the tee and I missed a couple of drives on the back nine and made a couple of bogeys and a bad double bogey there on 17," he said before accentuating the positives.
"I'm happy with my position. I felt like I played well today. A couple of loose drives and it's easy to make bogeys around here ... so if I make a mistake I don't let it bother me."
Chappell fired 68 to join Johnson at eight-under 202.
Johnson's stumble let a bunch of players back into the hunt, with Northern Irishman Rory McIlroy (66) and American Ryan Moore (66) two shots behind, and Japanese Hideki Matsuyama (68) three back despite a triple-bogey at the 14th.
Johnson, top seed in the FedExCup standings, will clinch the season-long points race and its $10 million bonus if he wins the tournament on Sunday.
But Chappell, 15th in the standings, is unlikely to claim the FedExCup top prize even if he wins the tournament. He is a three-time runner-up on the PGA Tour this year.
"I was so proud of myself the way I hung in there (and) I was able to turn it around (with a chip-in birdie at the 12th)," said Chappell, who has just one bogey in 54 holes.
McIlroy, sixth in the points list, gave himself a crack at the $10 million with three birdies in the last six holes.
"I knew DJ (Johnson) was a few shots ahead of me and if there's anyone I’m looking at this week, he’s the guy," said the former world number one.
"I’m going to need something similar if not better tomorrow. I shot a good one and that’s all I could do today."
South African Charl Schwartzel posted consecutive eagles, the second from 140 yards out at the par-four seventh, in carding 66 to vault within five shots of the lead.
(Reporting by Andrew Both in Cary, North Carolina; Editing by Larry Fine)
