DiMarco, who hasn't won on the PGA Tour since 2002, has been playing with arthritis, tendinitis and a bone spur in his left shoulder for about three months. Two recent cortisone shots and a slight adjustment to his swing seemed to ease his pain. The 64 was his best round of the season.
"It felt a lot better out there today,'' he said. "Three weeks ago, every shot I hit, whether it was a 70-yard lob wedge or whatever, it would pop.''
Five golfers finished at 65, including Tour rookie Michael Sim of Australia, who shot 29 on his closing nine.
"Once you get on a roll, you just make birdies,'' Sim said.
DiMarco said he tinkered with his putting grip, which seemed to help his short game. He needed just 26 putts.
"I made a bunch of good little putts today,'' he said. "When you make those, confidence breeds and you make longer ones.''
The best shot Thursday came from Craig Perks of New Zealand, who used a 6 iron to ace the 202-yard par-3 eighth hole. His blind tee shot went over the water and rolled into the cup.
Perks, who shot a 74 in the first round, took home a gold Rolex watch for his effort. Defending champion J.J. Henry, from nearby Fairfield, shot a 68.
Masters champion Zach Johnson shot a 71 and Vijay Singh a 68. Singh entered the tournament in third place in the race for the Fedex Cup. He has a chance to make up ground on Tiger Woods and Mickelson.