Sun City - Louis Oosthuizen said fatigue got the better of him in the second round at the Nedbank Golf Challenge on Friday.
The 2010 Open champion spent Wednesday at a hospital in Rustenburg treating a kidney stone, which almost resulted in him not teeing off in the opening round on Thursday.
However, Oosthuizen bounced back to take the overnight lead by three shots heading into Round 2.
On Friday, the Gary Player Country Club was not his friend this time around as Oosthuizen posted a level par round of 72 to remain on 9-under - two shots behind leader and fellow countryman Zander Lombard.
"I was fine starting, the last six holes got me a bit. The tee shot at the 14th, I felt pain in the right lower side and the next two holes, I was thinking about that and I didn't really focus on what I had to do," Oosthuizen told reporters on Friday.
"I could feel that I wasn't on song with the way I was hitting it. It's fatigue, I need to rest well for tomorrow."
Only one golfer managed to hit a bogey-free second round - Malaysian's Gavin Green - who shot a 2-under 70.
Oosthuizen said it was tough day and was bound to get tougher heading into the weekend.
"It was tough... it was guessing the whole day so it was difficult around this golf course. Other than Zander's round, I don't think there's low rounds out there," said SA's top ranked golfer.
"Hopefully this was my bad one for the week but I think the golf course is going to play really tough on the weekend.
"You could see the sort of pins and I think on the weekend we're going to have a proper test."
Oosthuizen will tee off alongside Belgian Thomas Detry and leader Zander Lombard at 10:20 on Saturday.
Leading scores after the second round of the European Tour's Nedbank Golf Challenge on Friday at the par-72 Gary Player Country Club in Sun City (RSA unless noted):
133 - Zander Lombard 68-65
135 - Louis Oosthuizen 63-72
137 - Thomas Detry (BEL) 66-71
138 - Tommy Fleetwood (ENG) 69-69, Marcus Kinhult (SWE) 69-69
139 - Joachim B Hansen (DEN) 69-70, Aaron Rai (ENG) 70-69, Jason Scrivener (AUS) 69-70
140 - Nacho Elvira (ESP) 70-70, Matthew Fitzpatrick (ENG) 71-69, Mikko Korhonen (FIN) 70-70, Guido Migliozzi (ITA) 67-73, Alvaro Quiros (ESP) 70-70, Paul Waring (ENG) 69-71, Bernd Wiesberger (AUT) 71-69