Mahan, who finished second here last year, shot an 8-under 62 Thursday to take the first-round lead at the Travelers Championship before play was suspended with 39 golfers still on the course.
The round was one stroke off the course record and it was his sixth consecutive round in the 60s at the TPC at River Highlands.
'It's one of the courses that you look at it and you feel comfortable,' he said. 'There's not really a shot here that I worry about.'
Mahan shot a 31 on his first nine holes, made par at 10, then birdied his next four holes.
A rain delay of over an hour didn't seem to phase him, as he came back out and birdied 17.
'I hit my driver great, put the ball in play,' he said. 'I hit a lot of good irons and wedges.'
That run allowed him to overtake early leader Chris DiMarco who shot a 64 despite an ailing shoulder.
'I hit a ton of good golf shots today,' Mahan said. 'But it's just the first round. We've got a long way to go.'
Mahan's round tied his best ever, which was in the third round of the 2003 Valero Texas Open. His previous best this year was a second-round 65 at the Buick Invitational. He's had two top-10 finishes this year, but has never won on the PGA Tour.
'If I can build on my score from today, I feel I have a pretty good shot,' he said.
The course record is 61, held by four players, including two-time champion Phil Mickelson.
DiMarco, who hasn't won on tour since 2002, has been playing with arthritis, tendinitis and a bone spur in his left shoulder for about three months.
He said two recent cortisone shots and a slight adjustment to his swing seemed to ease his pain, and allow him to focus on golf. 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, a 22-year-old from Australia, who shot 29 on his closing nine.
'You know how it is, 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 had 26 putts Thursday.
'I made a bunch of good little putts today,' he said. 'When you make those, confidence breeds and you make longer ones.'
When play was suspended for the rainy day around 7 p.m., 13 groups were still on the course. Sixty-two golfers were under par, a far cry from last week's U.S. Open, where there were eight sub-par rounds all week.
'This is what golf should be,' DiMarco said. 'It shouldn't be about plus 20s and plus 25s. I think the fans, if we started doing that every week, I think we would lose our fans. I think they want to see us make birdies.'
Craig Perks of New Zealand used a 6-iron to ace the 202-yard 8th hole. His blind tee shot went over the water and rolled into the cup.
Perks, who shot a 74, took home a gold Rolex watch for his effort.
Defending champion J.J. Henry, who is from Fairfield, shot a 68.
Masters champion Zach Johnson was at 71, and Vijay Singh came in at 68.
Singh came into 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, who both skipped the Travelers.