Mexico's Roberto Diaz holed a 100-yard wedge shot for eagle at his first hole and never looked back as he fired a flawless nine-under-par 62 and grabbed a two-stroke lead after the first round at the John Deere Classic in Illinois.
Americans Russell Henley and Adam Long were in second on 64, while Britain's Martin Laird, who is missing his home Scottish Open this week, was among a group of six players on 65 at TPC Deere Run in Silvis.
Ireland's Seamus Power shot a round of 69, leaving him on -2 in the clubhouse.
The best finisher on Sunday not already exempt for the British Open will earn the final seat on the overnight charter flight to Royal Portrush for next week's major championship in Northern Ireland.
Neither Diaz, Henley nor Long are currently in the Open.
Diaz, in his second year on tour, posted a career-best tie for eighth at the Travelers Championship three weeks ago, and the 32-year-old stepped up his game even further on Thursday.
"Everything clicked today," he told reporters of his round that included seven birdies.
"I've been playing pretty solid throughout the year. The driver has been awesome and I've been hitting a lot of fairways.
"It's awesome to get it going, and once you get it going, you just feel that you have to put the pedal to the metal and keep going because ... tomorrow it might be somebody else shooting 62 or 61."
Henley, meanwhile, lamented a bogey at his final hole, where his drive found a nasty lie in punishing rough, but he otherwise saw some promising signs.
"This has been a weird year for me," said the tour's 164th-ranked player.
"I just haven't felt myself playing golf. (I've been) a little lost but I've been putting in a lot of work on my game, and I felt confident going into today.
"I don't think I've ever lost the belief that I can have a nice tournament or a nice round. It's just a matter of a few bumps in the road here and there."
Long was a surprise winner at the Desert Classic in California in January, but has done precious little since.
He said there was a lot less stress thanks to the two-year exemption he received, but the game was the same challenge.
"It's still golf, it still drives you crazy, and I still have a bit of a perfectionist in me that I don't like to settle for mediocre," he said.
"I'm trying to get on my flight to Ireland here. I haven't qualified yet, so I'd love to be there."