Patrick Reed made what he called a "stupid decision" on his final hole Friday, and another earlier in his round.
The results? A two-stroke lead at 8-under-par 134 after two rounds at The Barclays.
"(Those) are my two mental mistakes where I went away from my game plan, cost me two shots," Reed said after carding a six-birdie 68 on day two of the FedExCup Playoffs kickoff at Bethpage Black in Farmingdale, N.Y.
The world No. 14 hit a driver off the 18th tee instead of a 3-iron, landing in a left-side bunker before bogeying the hole.
World No. 6 Bubba Watson found himself in trouble after taking the same strategy Thursday, losing a ball in the same left-side bunkers after his drive on No. 18.
On the par-three No. 8, Reed two-putted after placing his second shot 10 1/2 feet from the cup.
"I just need to get back to playing on the game plan," said Reed, who finished the day with three bogeys. "I did it perfectly for 18 holes yesterday."
Top-ranked Jason Day of Australia (70), world No. 3 Jordan Spieth (67) and 18-hole co-leader Martin Laird of Scotland (72) headline a tie for fifth with Sean O'Hair (69) and Canada's Adam Hadwin (68) at 138.
Reed opened with four birdies on his first seven holes before his first of three bogeys. After getting a birdie on No. 13 to drop to 9-under, he bogeyed No. 16 but birdied the next hole before a closing bogey.
The 33-year-old San Antonio, Texas, native and four-time PGA Tour winner is seventh in the FedExCup standings, but would rise to first with a victory at the challenging 7,468-yard Black Course at Bethpage.
Reed has three second-place finishes this season, although he hasn't won since the Tournament of Champions in January 2015.
"It's about consistency: Consistently getting in contention, consistently having chances to win," Reed said. "If I keep doing that, I will at some point break through and win hopefully a couple in a row, not just one."
Day held a brief share of the lead with Reed after recording five birdies through seven holes, but he bogeyed No. 8 and double-bogeyed No. 9. The reigning Barclays champion carded six birdies opposite three bogeys.
"It was a little difficult out there but I felt like I ground it out pretty good," Day said. "I know Patrick is up there, and I'm currently four back. Just got to try to get it under par and slowly chip away at it."
Spieth, who hoisted last year's FedExCup, got off to a rough start with an opening double bogey, but was clean after that with an eagle, four birdies and 12 pars. His eagle came at the 517-yard, par-five No. 4.
Steve Stricker (70), who won the 2007 Barclays, is tied for 61st at 144. Watson (74) is tied for 68th at 145.
Jason Kokrak and Jamie Lovemark had the low rounds of the day, posting 66s to rise into a tie for 18th at 140.
PGA champion Jimmy Walker (73) was among the 41 players who missed the 3-over cut, finishing at 147.
The top 70 players plus ties advanced to the weekend rounds.
NOTES: Robert Garrigus withdrew during Friday's second round with an elbow injury. Garrigus, the world No. 202, was 3-over through 11 holes after an opening 77. ... Players wore purple ribbons provided by world No. 73 Smylie Kaufman in support of Louisiana flood victims. "As an LSU graduate I've been deeply touched by this and so I'm doing what I can to help those who need all they can get right now," Kaufman said ... Jordan Spieth, who ranks second in the United States Ryder Cup standings, believes Scotland's Russell Knox deserves a spot on Europe's Ryder Cup team. "I don't think there's any doubt he deserves that pick," Spieth said. "I think it would be ridiculous if he didn't get one of the picks." Knox, who won the Travelers Championship on Aug. 7, ranks 30th on the European Points List and 10th on the World Points List.