His 62 tied the record set by Bart Bryant in the first round of the 2005 Tour Championship.
Harrington, in the first group off when the greens were at their smoothest, birdied his last four holes for a 63, while John Rollins rode a 3-iron into 20 feet for eagle on the 15th hole for a 64.
Woods managed to get off two shots a 3-wood to the fairway and a wedge to 8 feet when the sirens sounded to stop play, rain deluged East Lake and play was stopped until 5:20 p.m.
He returned to make three straight birdies, the final one from 30 feet after barely getting onto the green from the rough.
"That putt was bouncing all over the place," he said. "It bounced to the right. I thought I missed it. Then it bounced left. I thought I was going to miss it left. And then somehow, it wiggled back to the right up the hill and it went in."
The greens were almost dead three weeks ago because of record heat and no rain in the Atlanta area. Tour officials, with help from superintendents of neighboring golf clubs, did a noble job getting them ready for the FedEx Cup finale. They sodded some areas of the greens, tried to fill in barren patches with green sand and let the grass grow as much as possible.
"With all they've gone through, they've done a a great job to get them where they are," Clark said.
They were running considerably slower than most PGA Tour events, and players were predicting low scores even before the rain.
It didn't take long for them to be proven right.
Clark was 3 under after four holes, made a 20-foot par save on the par-5 ninth, then allowed a 59 to enter his thinking when he holed a chip for eagle on the 15th that put him at 8 under with three holes remaining.
"Unfortunately, they're not birdie holes," Clark said. "I was certainly trying, but I hit a few loose shots coming in. Overall, I knew if I parred the last few holes I was going to be pretty happy with the round."