Dustin Johnson moved into a five-shot lead in China on Friday after shooting a course-equalling nine-under-par 63 in the WGC-HSBC Champions in Shanghai.
Johnson birdied six of his first seven holes - his only par coming on the third - and narrowly missed further birdie chances on the eighth and ninth to be out in 30.
The big-hitting American dropped his first shot of the day after missing the green on the 10th, but responded with birdies on the 13th and 14th to reach 10 under par.
Johnson extended his lead with a birdie on the 16th and, when he made his 10th of the day on the last, the 29-year-old had equalled the course record with a superb 63.
Rory McIlroy began the day with a two-stroke advantage, but he could only shoot par in the second round meaning he is left in a three-way battle for second.
McIlroy is tied on seven-under with 2012 Masters champion Bubba Watson and former Ryder Cup player Boo Weekley, the American duo recording rounds of 69 and 67 respectively.
The Northern Irishman was still three clear when he followed five straight pars with birdies at the sixth, seventh and eighth but he struggled on the back nine.
McIlroy bogeyed the 11th after catching the face of a fairway bunker with his escape and dropped another shot on the 15th by failing to get up and down from a greenside bunker.
The 24-year-old scrambled pars on the next two holes before his drive on the par-five 18th stopped just a few feet from the lake which runs down the right-hand side of the hole, leaving him 228 yards to the green, all over water.
McIlroy had no hesitation in taking the shot on, but saw his second shot come up several yards short of the green in the hazard. After taking a penalty drop back onto the fairway, McIlroy hit a wonderful fourth shot to 15ft, but missed the par putt and signed for a level-par 72.
England's Tommy Fleetwood and Spain's Sergio Garcia were a shot behind on six under, with US Open champion Justin Rose and Open champion Phil Mickelson on five under.
Mickelson, who took nine on his penultimate hole on Thursday in an opening 71, birdied five of his last six holes in his 68, dropping a shot on the reachable par-four 16th after hooking his tee shot into the water.
Rose's water trouble came on the last, the Englishman thinning a bunker shot across the green and into the hazard on his way to a closing bogey and round of 71.