McIlroy moved in front of the FedEx Cup playoff race last week by winning the Deutsche Bank Championship and his 8-under-par 64 Thursday left him tied at the top with Bo Van Pelt, Graham DeLaet and U.S. Open champion Webb Simpson.
The 70 players in the field were able to take advantage of the lift, clean and place rules after the course became sodden this week by the remnants of Hurricane Issac.
McIlroy, having backed up his recently earned No. 1 world ranking with outstanding play, eagled the par-5 15th to highlight his round.
"With the soft conditions you could really shoot a number out there, and I did," McIlroy said. "I missed a few putts, but I was able to make most of them. Great start to the tournament."
Defending champion Justin Rose shot a 5-under 67 Thursday and was in a tie for 10th place.
The 70 players left in the FedEx Cup race will be trimmed to 30 for the Tour Championship, which will be played in two weeks in Atlanta.
The FedEx Cup champion will earn $10 million and it has been shown that just about anybody who makes the Atlanta field can win the top prize. Bill Haas began the Tour Championship in 25th place on the points list last year and wound up No. 1.