Bo Van Pelt shot an opening six-under par to lead the Barclays by one shot at the Ridgewood Country Club in New Jersey.
Van Pelt chipped in on the par-five 17th for a magnificent eagle to move clear of a large chasing pack just one shot behind the leader.
The American, who was ranked 104th heading into the event, admitted afterwards that he didn't actually make a good connection with his chip shot from just off the front of the green. He believed that he had left it slightly short but the ball rolled on down the slope before dropping in for three.
The 39-year old believed his round could have been even better if he had put away a couple more opportunities but was glad with the way he finished.
"Through the middle I played well but I just didn't convert on a couple of opportunities," he said. "Probably the key was on the 15 - I hit a poor shot but then made par. I then went three-three on 16 and 17 to finish the round."
Hot on the trail of Van Pelt are Ben Martin, Hunter Mahan, Jim Furyk, Brendon Todd, Charles Howell III and Cameron Tringale. All in the large pack behind their compatriot on five under par, alongside England's Paul Casey and Zimbabwe's Brendon De Jonge.
Ryo Ishikawa of Japan, Australian Danny Lee and Scotland's Richard Knox, whose only bogey of the day came on his final hole, are all just a shot further back.
Americans Rickie Fowler and Bubba Watson are both on three-under par alongside South African Ernie Els.
Englishman Justin Rose also sits on three-under par but bogeys on 12 and 17 meant he could have been carded a better score than 68.
But Rose is really pleased with the way he's playing at the moment, especially with some of the putts he made during his opening round.
“My game feels close,” Rose said. “I played really well at the PGA as well but my putter let me down so I was really pleased today that there were some four, five putts that I got and that kept my momentum going. It was something I struggled with at the PGA.
“My ball striking was there or thereabouts. I didn’t hit that many fairways today so I was happy to get round.”
Australia's Adam Scott is on two under, with Graeme McDowell and Lee Westwood a shot further back. Phil Mickelson carded a 71 level-par along with European Ryder Cup trio Sergio Garcia, Luke Donald and Martin Kaymer.
The top 125 players in the FedEx Cup standings qualify for the Barclays with 100 players progressing to the Deutsche Bank Championship.
Only 70 players go onto the BMW Championship, with the field being cut to 30 for the Tour Championship. The player finishing top of the standings after the Tour Championship claims a $10 million bonus.
McIlroy led the way coming into the tournament, followed by Jimmy Walker, Tom Watson, Matt Kuchar and Jim Furyk.