Dustin Johnson fired a bogey-free 63 to grab a one-shot lead at the halfway stage of the WGC-HSBC Champions in Shanghai.
The world No 1, looking to become the first player to win three World Golf Championship titles in a calendar year, struck nine birdies in a flawless display at Sheshan International Golf Club.
Johnson birdied six of his final seven holes to get to 13 under and move ahead of overnight leader Brooks Koepka, who followed up his opening-round 64 with a four-under 68.
Justin Rose and Kiradech Aphibarnrat sit four strokes back in a share of third, with Patrick Reed holding fifth spot and Matt Fitzpatrick in the group six off the pace.
Koepka doubled his overnight lead after following a 20-foot gain at the first with a two-putt birdie at the next, only to par his next seven holes to reach the turn one ahead of Aphibarnrat and Rose.
Johnson picked up a shot at the second and added birdies at the eighth and 10th, before back-to-back gains from the 12th lifted the American within one of the lead.
As Rose and Aphibarnrat both bogeyed the last to set the initial clubhouse target at nine under, Johnson took the outright lead by following a 15-foot gain at the 15th with another birdie at the next.
Johnson holed from eight feet at the 17th and pitched to leave another birdie at the last and close a back-nine 29, only for Koepka to close the gap along the closing stretch.
The US Open champion nudged in from close range at the 16th and drained a 25-footer at the next, but pitched to six feet and missed a birdie putt for the share of the lead at the last.
Rose mixed six birdies with two bogeys in a four-under 68, while Reed moved up the leaderboard with four gains in a six-hole stretch on his blemish-free back nine.
Fitzpatrick is joined on seven under in tied-sixth by Henrik Stenson, Brian Harman and Matt Kuchar, with Rafa Cabrera Bello and Tyrrell Hatton part of the group a further shot back.
Defending champion Hideki Matsuyama lies in a share of 66th after a second successive 74, with Jon Rahm also over par for the week.
