Shanghai - World number one Dustin Johnson nailed four birdies in his last four holes to seize a one-shot lead at the halfway stage of the $9.75 million WGC-HSBC Champions in Shanghai on Friday.
Johnson's spectacular nine-under-par 63 catapulted him over fellow American and overnight leader Brooks Koepka, who hung on for second on 132 overall.
Also in the hunt was the Englishman Justin Rose, the Rio 2016 Olympic champion, and Thailand's Kiradech Aphibarnrat. They shared third on 135 at "Asia's major".
Fifth was Patrick Reed of the United States, five shots off the lead.
Johnson, who triumphed in Shanghai in 2013 and has won all four prestigious World Golf Championships (WGC), followed up his opening 68 with a sparkling round that started well and only got better as the day wore on.
The 33-year-old had a couple of near birdie misses early in his second round, but found his range with back-to-back bridies on holes 12 and 13, then ramped up his challenge to finish with a flourish.
Johnson, tied 11th after the opening round, said he had switched putters less than 20 minutes before his second round - and it paid off in a big way.
"I wasn't really pleased with the one I played with yesterday so the guys here in China made me one," said the 2016 US Open champion.
"I got it right before I walked to the tee. I hit a few putts on the practice green with it and I was like, 'Oh, this will work pretty well'.
"Went out and holed a lot of putts today, so kind of like it."
Koepka, whose first major title came at the US Open in June, stayed in contention with a solid 68 at a sunny Sheshan International Golf Club.
The 27-year-old began his second round with real intent, reeling off two birdies on the trot to start the second day much as he finished the first.
Roses's second round would have been even better but for a bogey at the 18th and he carded a 68 to go with his first-round 67.
"Listen, it's part of the 18 holes," Rose said of his last-gasp blip.
"Disappointing way to finish, felt like I was running out of steam a little bit coming in."
But defending champion Hideki Matsuyama, the world number four from Japan, again failed to fire.
He followed up his disappointing opening-round 74 with another two-over 74 as his challenge disintegrated.
Leading second-round scores at the $9.75 million WGC-HSBC Champions in Shanghai on Friday (par 72):
131 - Dustin Johnson (USA) 68-63
132 - Brooks Koepka (USA) 64-68
135 - Justin Rose (ENG) 67-68, Kiradech Aphibarnrat (THA) 65-70
136 - Patrick Reed (USA) 66-70
137 - Matthew Fitzpatrick (ENG) 68-69, Matt Kuchar (USA) 67-70, Brian Harman (USA) 68-69, Henrik Stenson (SWE) 68-69
138 - Rafael Cabrera Bello (ESP) 68-70, Tyrrell Hatton (ENG) 68-70
139 - Peter Uihlein (USA) 72-68, Kyle Stanley (USA) 71-68, Tony Finau (USA) 67-72, Daniel Berger (USA) 68-71, Wu Ashun (CHN) 67-72, Gavin Green (MAS) 65-74, Charles Howell (USA) 72-67, Scott Hend (AUS) 71-68, David Lipsky (USA) 69-70, Matthew Southgate (ENG) 68-71
140 - Paul Dunne (IRL) 67-73, Ryan Fox (AUS) 71-69, Haydn Porteous (RSA) 66-74
141 - Pat Perez (USA) 72-69, Bernd Wiesberger (AUT) 71-70, Chan Kim (USA) 69-72, Tommy Fleetwood (ENG) 71-70