Los Angeles - World number 22 Bryson DeChambeau finds himself in a race to regain fitness for next week's British Open after injury brought his John Deere Classic title defence to a premature halt on Thursday.
DeChambeau, who won the Jack Nicklaus-hosted Memorial in June, hurt his right shoulder hitting out of the rough at the second hole at TPC Deere Run in Sylvis, Illinois - where journeyman Steve Wheatcroft was the surprise leader.
The 24-year-old Californian played on, shaking out his arm to relieve the discomfort on the 12th tee before calling it a day after hitting his tee shot at 15.
On-site medical staff told him there was some "instability" in the shoulder joint.
"On two, I hit the shot out of the rough on the right and I just didn't feel great after that," DeChambeau said. "I was feeling great coming into today."
He was off to consult a doctor, still hoping he could be ready to tackle Carnoustie in the third major of the year next week.
"If I can get three or four days of good rest in, when I get there Monday night I'll evaluate it and see how I feel," he said.
Despite his career-best round of nine-under par 62, Wheatcroft wasn't allowing himself to think about a trip to Carnoustie.
Asked if he had his passport with him, just in case he hangs on for a first US PGA Tour title that would earn him a late ticket to the Open Championship, Wheatcroft said he did - but only because he always travels with it.
In the midst of a tough season, he wasn't getting ahead of himself.
"It's been a brutal year, to be honest with you," said Wheatcroft, the 40-year-old, ranked 927th in the world.
"It's probably the worst year I've had so far, but I don't feel like I'm playing the worst golf of my career. I actually feel like I'm playing some good golf, I'm just not getting the results out of it.
"You know, I feel like it's close - 62 didn't surprise me."
Wheatcroft reeled off six straight birdies to finish his round. He was one stroke in front of fellow American Michael Kim.
Canadian Nick Taylor, Argentina's Andres Romero and Americans Johnson Wagner and Joel Dahmen shared third on 64.
Wheatcroft took full advantage of ideal morning scoring conditions. With crisp iron play he left himself birdie putts of six feet or less at holes 13, 14 and 15 to key his finishing burst.
He said a new driver head, which he had only tried out on the driving range, was a big help as he found 12 of 14 fairways.
After storming home, Wheatcroft said: "I wanted to just go to the first tee and start the second round."
Leading first-round scores on Thursday in the PGA Tour John Deere Classic in Sylvis, Illinois (USA unless noted, par-71):
62 - Steve Wheatcroft
63 - Michael Kim
64 - Johnson Wagner, Nick Taylor (CAN), Andres Romero (ARG), Joel Dahmen
65 - Scott Brown, Francesco Molinari (ITA), Kim Whee (KOR), Denny McCarthy
66 - Patrick Rodgers, Kelly Kraft, Tyler Duncan, Parker McLachlin, Corey Conners, Fabin Gmez (ARG), Chad Campbell, Matt Atkins, Nick Hardy, John Merrick, Chris Kirk, David Hearn (CAN), Kevin Streelman, Sam Ryder, Lanto Griffin