Louis Oosthuizen opened a five-stroke lead in China over Adam Scott and Ernie Els, but former US Open winner Graeme McDowell could drop out after injuring his hand.
Oosthuizen opens lead at WGC Champions as freak injury puts McDowell's season in doubt.
Louis Oosthuizen picked up six shots in just five holes on the way to a red-hot 63 and a big five-stroke lead in the $7 million (Dh25.7m) WGC-HSBC Champions today.
An eagle on the par-five seventh sparked a blazing run from the 2010 British Open winner, who birdied his next four along with 15 and 16 for a nine-under-par round and a total of 16-under 128 at southern China's par-72 Mission Hills.
Oosthuizen's round, blotted only by an early bogey on the second, opened up a healthy advantage over a top-quality chasing pack led by Adam Scott and Ernie Els, who were tied on 11 under par.
"You can see there's a lot of birdies out there with five par-fives," said Oosthuizen, who was greeted at the clubhouse by his wife and young daughters, who are travelling with him.
"But I'm making it easy for myself — I'm hitting a lot of good shots. It's far from over, there's a lot of golf left. I need a good round tomorrow to get myself in position for Sunday."
Four-time Major-winner Els turned at seven under par and added two more birdies on his way back for a 63, matching his fellow South African Oosthuizen, while Scott shot 68.
Phil Mickelson, the event's only double winner, was set for a share of second place until he found greenside rocks on the 18th for double-bogey, dropping down to tied sixth with Dustin Johnson.
Jason Dufner, Mickelson and Johnson's American Ryder Cup teammate, shot 66 to share fourth spot with Ireland's Shane Lowry, who had 68.
Scott, who had shared the first-round lead with Oosthuizen, played a sumptuous front nine studded with four birdies but he stumbled on his return with three bogeys and another three birdies.
"I played OK today — I got off to a good start but on the back nine I missed a few greens and I had to scramble, and it's tough when it's fast around the greens," said Scott.
The Australian escaped from a bunker on the third hole for his first birdie of the day, and he picked up further shots on four and five to take a two-shot lead early in the round.
A inappropriate cry of "Adam, we love you!" from two female marshals spurred him on, but it was Oosthuizen who then took charge with his eagle on the par-five seventh, cueing up a birdie blitz around the turn.
Oosthuizen birdied the par-three eighth, escaped out of a bunker to within a foot on nine, drained a 20-footer on 10 and then chipped to an unmissable distance on 11.
Two more birdies on 15 and 16 completed a pleasing day's work for the 30-year-old, who now has great chance of recording his sixth win and the second biggest of his career, after the 2010 Open.
Meanwhile former US Open winner Graeme McDowell's season could be at an end after he slammed his hand in a hotel door.
McDowell battled through his round at the WGC Champions after the freak accident, but will decide whether to continue at the tournament after medical tests on Saturday.
The Northern Irishman's manager, Conor Ridge, said McDowell got his right hand caught in the door just before leaving his hotel for the second round at southern China's Mission Hills.
McDowell looked in pain during his round of 75, which left him tied for 56th. He has three events left on this year's schedule: the World Tour Championship in Dubai, the Australian Masters and the World Challenge in California.
"He got it treated this morning. It was obviously quite sore because he said it feels like it's broken," said Ridge. "They've confirmed since that it's not broken, it's just bruised.
"He did it this morning before he left the hotel room, literally when he was on his way down here so we're going to see what it's like in the morning."