CROMWELL, Connecticut, June 21, 2012 (AFP) - Australian Nathan Green, Sweden's Fredrik Jacobson and American Will Claxton shared second on 65 with 10 rivals another stroke adrift, including new US Open champion Webb Simpson and US compatriot Bubba Watson, the 2012 Masters winner.
Mathis, who began on the back nine, opened with a birdie and then aced the par-3 11th and followed that up with birdies on three of the next four holes to reach six-under after six holes.
He answered a bogey at 18 with a birdie at three and a bogey at the par-3 eighth by closing with a birdie to sit atop the leaderboard for the first time since last year's opening round at Memphis.
"I was hitting a lot of great shots," Mathis said. "To make a hole-in-one at 11 and a long putt on 12 just really got me jump-started for the day."
The ace was his shot of the day.
"We were just trying to cut a 9-iron in there to that back right pin and I hit it just pretty much perfect where we were looking to hit it. It kind of bounced a little right and went right in the hole. It was cool."
Mathis, who missed the cut in 10 of his first 11 events this season, isn't thinking about a title just yet, however.
"I've been in this spot before, but there's so much golf left, you're just thankful to play a solid round. It's all about trying to build on it and being there toward the end and giving yourself an opportunity to win."
Simpson, coming off his first major title last week at The Olympic Club in San Francisco, decided to play this week as he planned to repay the loyalty shown to him in 2008 as a young player out of college when sponsors gave him an exemption to participate in the event. 
Sponsors paired him with Watson and 2011 PGA Championship winner Keegan Bradley in a trio of reigning major champions from the United States.
"It was fun," Simpson said. "It took a lot of pressure off me because I'm not the only stand-out guy in the group. It did relieve a little stress."
Simpson opened with a birdie and added another at the par-5 sixth. He had back-to-back birdies at 12 and 13 and another at the par-4 15th before his lone bogey of the day at the par-3 16th.
"I'm definitely pleased," Simpson said. "I knew it was going to be a tough round to come out and really focus and put last week behind me.
"But we got off to a good start as a group, two of us birdied No. 1, and it was just fun playing with those guys. We just kind of fed off each other and just played a good solid round."
And for the first time, Simpson was introduced as a US Open champion.
"That was pretty cool on the first tee," Simpson said. "I never really dreamed that I would hear that, but when I heard it, I kind of smiled inside."
Leading scores on Thursday after the first round of the $6 million US PGA Travelers Championship (USA unless noted):
64 - David Mathis
65 - Nathan Green (AUS), Will Claxton, Fredrik Jacobson (SWE)
66 - Jerry Kelly, Gary Christian (ENG), Roland Thatcher, Tim Clark (RSA), Rocco Mediate, Webb Simpson, Bubba Watson, Chez Reavie, Tommy Gainey, Kevin Kisner
67 - Brandt Jobe, Ken Duke, Chris DiMarco, Greg Chalmers (AUS), Charley Hoffman, Matt Kuchar, Roberto Castro, Brian Davis (ENG), Cameron Tringale, Aaron Baddeley (AUS)