Defending champion Vijay Singh posted a 5-under-par 67 to share third place with Tim Clark, Tom Pernice, Jr. and J.J. Henry.
DiMarco, who is competing for the first time since his playoff loss to Tiger Woods at the Masters, played the back side first at the TPC of Louisiana and picked up his first birdie of the day at the par-4 12th after his second shot stopped within 15 feet of the hole.
'I knew that I was swinging great after the Masters,' said DiMarco. 'Right now it's pretty simple. I'm not really thinking about it too much. I'm just trying to think about what shot I'm trying to hit into the green, and I'm not getting ahead of myself.'
He hit a 6-iron to 3 feet for a birdie at the par-4 16th and added a birdie at the par-5 18th to make the turn at 3 under. DiMarco then played his approach to 7 feet at the par-4 first and converted the putt for another birdie.
'I was able to really control my ball flight today,' he said. 'My distance control was very good.'
At the par-5 seventh, DiMarco roped a 3-wood inside 20 feet and drained the eagle putt. DiMarco then hit a wedge to 8 feet for a birdie at the par-4 eighth and walked off with a par at the ninth to secure a share of the lead.
'My confidence was very high going in,' said DiMarco, who posted his low round of the year on the PGA Tour. 'It obviously feels very good to shoot a 65 on this golf course.'
Atwal also started on the 10th and collected a pair of birdies over his first nine holes. Atwal then birdied two in a row from the first but gave a shot back with a bogey at the par-3 third.
The 32-year-old responded with a birdie at the fourth and ran off three consecutive birdies starting at the par-4 sixth to join DiMarco atop the leaderboard.
'I started just trying to make pars off the 10th hole because it was so windy,' said Atwal. 'I was putting it pretty well. It was a good putting day basically.'
Singh played the back nine first and tallied two birdies and a bogey over his first four holes. Singh dropped another shot with a bogey at the 16th but he rolled in a lengthy birdie putt at the par-5 18th to reach minus-1.
Singh missed the green en route to a bogey at the first but he countered with a birdie at the very next hole. He kept on rolling with a birdie at the fourth and sank a 20-footer for a birdie at the par-4 sixth.
He made it two in a row with a birdie at the seventh and left his drive within 30 yards of the green at the par-4 eighth. Singh holed out for an eagle but he found trouble with a bogey at the ninth to finish two shots back.
'I thought I played pretty good,' said Singh, who won his third Houston Open last week. 'I played decent. I didn't play great. I hit a lot of good shots and capitalized on nice breaks.'
PGA Tour rookie Sean O'Hair also bogeyed the ninth to close out a round of 68. O'Hair finished alongside Padraig Harrington, Ryuji Imada, Nick Watney, Darron Stiles, James Driscoll and Joey Snyder III in a tie for seventh.