Charlie Beljan secured the halfway lead in the Children's Miracle Network Classic in Florida before being taken away to hospital.
The 28-year-old rookie suffered shortness of breath and an elevated heart rate during his second round at the Palm Course but still managed to card a superb eight-under 64 and secure a three-shot lead at the halfway mark.
Beljan started the season's final event 139th on the US Money List and is battling to make the top 125 who secure their cards for next season.
He made the perfect start with an eagle at the opening hole and added another at the 11th to be eight-under for the day and 12-under overall.
However, as his health deteriorated - he dropped to his knees on several occasions - he mixed two birdies and two bogeys over the last seven holes and medical personnel walked with him for much of the back nine.
"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," Adcox added. "I thought he was going to quit out there a few times ... Unbelievable."
Behind him there's a huge log-jam with seven players sharing tied second on nine-under and a further 10 on eight-under.
Sweden's Henrik Stenson looked set to be Beljan's nearest challenger but bogeyed his final hole in a round of 67 to drop him into a tie with Americans Harris English, Charles Howell III and Mark Anderson, Japan's Ryuji Imada, Australian Matt Jones and first round leader Charlie Wi from Korea.
Scotland's Russell Knox could only manage a level par 72 at the Magnolia Course, which will be used for the final two rounds, and is now six back in tied 21st.