Bubba Watson charged into a tie for the first-round lead at The Barclays while newly crowned world number one Jordan Spieth struggled to his worst score of the year in relation to par.
Twice Masters champion Watson used a red-hot outward nine as a springboard en route a sizzling five-under-par 65 at Plainfield Country Club, setting the pace with fellow Americans Spencer Levin and Tony Finau, and Colombia's Camilo Villegas.
New Zealander Danny Lee, Americans Jason Dufner, Cameron Tringale and Bryce Molder, and Britons Paul Casey and Russell Knox opened with 66s in the first of the PGA Tour's four lucrative FedExCup playoff events.
Spieth, however, was let down by his usually stellar short game as he limped to a 74 in his first competitive round since he replaced Rory McIlroy at the top of the rankings by finishing second at the PGA Championship 11 days ago.
"After the tee shots, that is the worst round I've played in years," Masters and US Open champion Spieth, who hit 11 of 14 fairways on a challenging Plainfield layout, told reporters.
"It was really a very unique round for this year. I hit my first 11 fairways today and that's really hard to do out here."
World number three Day, who withdrew from Wednesday's pro-am competition due to a back issue, opened with a 68.