Charlie Beljan defied a health scare to take a halfway three-stroke lead in the the Children's Miracle Network Hospitals Classic in Florida.
The American, who suffers from asthma, had to rest on several occasions during his round after battling shortness of breath and an elevated heart rate.
The 28-year-old was taken to hospital after recording a magnificent 64, which moved him to 12 under par overall, but was still expecting to play on day three.
Overnight leader Charlie Wi was among seven players three strokes behind Beljan.
The Korean, without a win on the PGA Tour, failed to build on yesterday's magnificent 64 after recording a one-under-par 71 today.
He enjoyed mixed fortunes on the Magnolia course with five birdies and four bogeys.
Australia's Matt Jones put himself into contention with an excellent round of 64 to join Wi, Ryuji Imada, Harris English, Charles Howell III, Mark Anderson and Henrik Stenson on nine under overall.
"He kept saying he thought he was going to die" - Beljan's caddie Rick Adcox
"I think he's scared," Beljan's caddie Rick Adcox told the Golf Channel. "He kept saying he thought he was going to die.
"I've got to give Charlie credit for what he did. I thought he was going to quit out there a few times. Unbelievable.
"A couple of times I thought he might pass out.
"He just said, 'I'm gonna keep going until I pass out or they take me off,' and I kept saying 'it doesn't matter to me, it's only a golf tournament'."
Later, Beljan was in positive mood when he spoke to the Golf Channel.
"As long as they tell me I'm not going to fall over and die," he said when asked if he would play on Saturday. "I'm hoping it was just a panic attack."
Beljan was reportedly struggling throughout, with medical personnel keeping a close watch on him for much of his back nine.
He was eight under for his round after 11 holes, carding two eagles and four birdies.
He then bogeyed the par-four 12th before picking up back-to-back shots on his next two holes although he dropped back to eight under for his round at the 17th.
Despite clearly struggling, he courageously carried on and managed to hit a brilliant up-and-down for par on the final hole before bursting into tears.
"He was trying to keep upright," playing partner Ed Loar said. "Hopefully he'll be all right. It was pretty bizarre."