Louis Oosthuizen shot eight birdies and an eagle to take charge of the WGC-HSBC Champions in Shenzhen, China.
The South African was the joint overnight leader with Australian Adam Scott but his nine-under round moved him five clear of Scott and Ernie Els.
Els birdied six of the first seven holes to tie Oosthuizen's course-record 63 and join Scott on 11 under.
"I'm in a great position to win, but it's not even crossed my mind at the moment," Oosthuizen said.
"There's still a lot of golf to be played - I need to put myself in a great position going down the back nine on Sunday."
Thirteen of the world's top 20 players are competing in Shenzhen, although world numbers one and two, Rory McIlroy and Tiger Woods, are absent.
Oosthuizen is now 16 under overall after recovering from a bogey on the second to birdie the third and fifth before an eagle on the 566-yard seventh put him in position to move clear of the field on the back nine.
Ireland's Shane Lowry and Jason Dufner, of the United States, are tied for fourth, six shots off the lead, while England's Luke Donald went around in 68 for the second successive day to move into a tie for eighth.
Donald's compatriots, Ian Poulter and Lee Westwood, are tied in 12th after advancing their overall score to seven under.
It was a bad day for Sweden's Peter Hanson, who is hoping to overhaul world number one McIlroy at the top of the European Tour money list.
He recovered from a bogey on the first to post an eagle on the third but a double bogey at the 12th saw him finish his round one under. That moved him from an overnight position of a tie for third to a tie for 14th alongside Poulter and Westwood.
Englishman Justin Rose, another man who can overhaul McIlroy with victory, is down in a share of 31st on two under.