American Kramer Hickok and Japan's Satoshi Kodaira posted matching rounds of seven-under-par 63 to share the first-round lead at the Travelers Championship in Cromwell.
The 63s marked a career-best round for Hickok and tied Kodaira's best on Tour. The pair have a one-stroke lead over Talor Gooch, who carded a bogey-free, six-under 64.
Kodaira's 63 matched his best score on the PGA Tour, from the second round of the 2018 RBC Heritage – his only victory.
He was helped when he holed out from the fairway for an eagle on the 348-yard, par-4 second.
"I knew it was on target but didn't see it go in," he said. "I heard the applause, so I knew it went in."
Hickok's top finish is a tie for eighth at the Bermuda Championship this year. He opened with a bogey on the par-4 10th, then rattled off eight birdies without dropping another stroke the rest of the way.
He missed a five-foot birdie putt on the 13th hole, but then birdied five of the next seven holes and he played the last four of the day in three under.
"I really only hit one bad putt – on the par-5 13th. Other than that, I just felt like the holes looked awfully big today," he said. "I made a lot of good putts today."
Hickok said he had his worst warmup session of the year in the morning, spraying the ball all over the range.
"So I really changed my strategy," he said. "You just take a lot weight off your shoulders and you just go and simplify things ... just saying, 'I don't have that shot today. I'm not going to try and hit it.' Just go with what you got."
Cameron Smith, after a three-under 67, heads an Australian contingent who are all even par or better.
After a disappointing US Open where he missed the cut, Smith put together five birdies and two bogeys to leave him tied 15th.
Marc Leishman, Adam Scott and Jason Day had one-under 69s, a stroke ahead of Cameron Percy, Matt Jones and Lucas Herbert.
Defending champion Dustin Johnson, who held the World No. 1 ranking until Jon Rahm passed him on Sunday with his US Open victory, stumbled at the start with a bogey on the second hole and then a double on No. 3.
He finished with a pair of birdies to end the day at 70.
Two-time Travelers champion Phil Mickelson sank a 64-foot putt for birdie on the par-4 ninth hole – his second-longest putt since the Tour began tracking such things in 2004.
The 2001 and '02 winner then made a 40-footer for par on No. 10.