David Mathis carded a 6-under 64 on Thursday to take a 1-stroke lead over three players after the first round of the Travelers Championship, though a number of big names aren't far off his pace.

Defending champion Fredrik Jacobson fired a 5-under 65 and shares second place with Nathan Green and Will Claxton.

Behind the second-place trio is a group of 10 knotted for fifth at minus-4. Among them are Webb Simpson, last week's U.S. Open winner, and Masters champion Bubba Watson.

They're all in pursuit of Mathis, who is chasing his first PGA Tour title. A two-time winner on the Nationwide Tour, Mathis has missed the cut 12 times this season, though does have a top-10 finish.

But Thursday, he got off to an incredible start, with a birdie at the 10th -- his first -- before an ace at the par-3 11th.

"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, and kind of bounced a little right and went right in the hole," Mathis said.

He followed that feat up with a birdie at 12, where his approach settled nearly 50 feet from the cup before he rolled in the long putt. Consecutive birdies at 14 and 15 pushed Mathis to minus-6 before he even got through nine holes.

But Mathis' momentum slowed. He bogeyed 18 before making the turn, and his front nine was more about maintaining his score than going further under par in the increasingly hot conditions at TPC River Highlands.

He birdied the third, and a bogey at eight threatened to push him back to the field as he went into the clubhouse. But he closed with another birdie to take first place by himself.

"It's all about trying to build on it and being there toward the end and giving yourself an opportunity to win," Mathis said. "So you know, it's a good start and just want to build on that."

Jacobson actually threatened to do Mathis one shot better. He made seven birdies through his first 15 holes and was in front of the tournament, but a double-bogey at 16 took him out of the lead, and he parred his final two.

Still, Jacobson considers his first round a good start to his title defense.

"I struck it really solid all day and really only had one hiccup on 16 where I landed the ball a foot from the hole, and ended up with a 5 to double there, but the rest was no mistakes," Jacobson said.

Like Mathis, Green got off to a good start on the back nine. He poured in five birdies and stumbled to one bogey, making the turn at 4-under. But his front nine was all pars until he birdied the last.

Claxton opened with a birdie and eagle to jump to minus-3, then birdied the ninth to bring himself further up the leaderboard. But three back-nine birdies were accompanied by two bogeys, as he picked up only one more stroke.

Simpson and Watson were playing in the same group, along with PGA Champion Keegan Bradley, who posted a 2-under 68.

Simpson showed few aftereffects of his U.S. Open title, carding two birdies and no bogeys on his front nine. He then drained three birdies in a four-hole stretch from the 12th to reach 5-under, though a bogey at the next hole put him into a tie for fifth.

"It was I think good for my mind to get out and get in the ropes again and start another tournament," Simpson said. "And in a way I had to put last week behind me and try to play as well as I could."

Watson was minus-1 through six holes before he went on a run. He birdied seven and nine, then eagled the 10th. But he, too, bogeyed 16 to sink to minus-4.

NOTES: Joe Ogilvie fired a 4-over 74 and released some alarming news after his round. He tweeted that he was going "straight to the hospital for MRI, herniated disk L5-S1. May have played my last round on the PGA Tour for 2012.