Louis Oosthuizen hauled himself from his sick bed to produce a thrilling 63 and set the pace at the Nedbank Challenge in Sun City.
Struggling with kidney stones, Oosthuizen had to miss Wednesday's pro-am, instead spending a chunk of the day in hospital.
But the 37-year-old South African rejected thoughts of abandoning the tournament and hit nine birdies in a blemish-free round on Thursday.
He revealed the secret to his success was a go-slow approach, ensuring he limited his pain levels and could focus on his swing
"Considering at 3am this morning I didn't think I was going to tee it up, I'm very chuffed with that round," said Oosthuizen, who sits three strokes clear of nearest rival Thomas Detry.
He said he had been toiling "all of yesterday with kidney stones and this morning was very uncomfortable".
"You could see me walking slower and doing everything a little slower out there," he added.
"All of yesterday was very uncomfortable – I spent a bit of time in the hospital in Rustenburg.
"This morning was just a little niggly, but I went up to the range to hit a few shots and it didn't really bother me hitting - it was more the walking fast that got it, so I just slowed everything a little down and it worked nicely."
Oosthuizen has finished in the top 10 in three of his last four outings at the Gary Player Country Club, which stages this event.
The 2010 Open champion is also looking to add to his victory last December at the South African Open and he opened up a healthy lead on day one.
Italy's Guido Migliozzi held down third place after a five-under 67, one ahead of South Africans Ernie Els and Zander Lombard and England's Lee Westwood.
Four-time major winner Els, at 50, rolled back the years with four birdies and no bogeys.
"I didn't quite expect that. I haven't played too much golf recently," Els told the European Tour website. "I really came here with a plan to try and get a swing going, which I kind of did.
"I just went on memory, going around the golf course where I used to hit the ball. Today was a good day."
Tommy Fleetwood, whose ball was meddled with by mongooses on the 17th green, was one of 10 players to sign for a 69, leaving him three under.