McIlroy was joined by Englishman Brian Davis and Australia's Geoff Ogilvy on seven under par as Jason Day and Zach Johnson set the pace with rounds of 63.
Tee-off times in the opening round of the second event of the FedEx Cup play-off series were brought forward and precautions were being taken to avoid damage in case hurricane Earl hit.
The edge of the storm, which was making its way up the east coast of the United States, brought lightning with it, forcing a 65-minute suspension.
However, with lift, clean and place in effect due to the wet conditions, there were numerous low scores.
Swede Fredrik Jacobson and England's Luke Donald were in a tie for 11th on six under, with Vijay Singh, Paul Casey and Martin Laird all a shot further back.