A new putter helped world number one Dustin Johnson produce a stunning burst of scoring to claim the halfway lead in the WGC-HSBC Champions in Shanghai.
Johnson birdied six of the last seven holes at Sheshan International in a brilliant, bogey-free 63 to reach 13 under par, a shot ahead of first-round leader Brooks Koepka.
Koepka also kept a bogey off his card as he added a 68 to his opening 64, leaving the US Open champion three clear of Olympic gold medallist Justin Rose and Thailand's Kiradech Aphibarnrat.
A victory for Johnson on Sunday would make him the first player to win three World Golf Championship events in the same year after he won the WGC-Dell Technologies Match Play and WGC-Mexico Championship on back-to-back starts in March.
"O bviously I played very well," Johnson told reporters after covering the back nine in 29. "But I actually got a putter like about 20 minutes before I teed off. It's a (TaylorMade) Spider, so it's one that I've used before.
"I wasn't really pleased with the one I played with (on Thursday), so the guys here in China made me one. 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'.
"I went out and holed a lot of putts, so I kind of like it."
Johnson also clearly likes the course at Sheshan International, where his 24-under-par winning total in 2013 included a second round of 63.
"It suits my eye very well," the 33-year-old added. " For me, if I drive it good out here, I feel like I can make a lot of birdies.
"I can reach all the par fives if I drive it in the fairway. The rest of the holes, I feel like if I'm in the fairway, I can attack the golf course.
"It's always important to get a win no matter what time of year it is. For me, it's kind of the end of the year but it's also the start of the 2018 (PGA Tour) season. It would be really nice to get off to a good start."
Johnson and Koepka are good friends and neighbours in Florida, although most of their time together is spent relaxing or in the gym, rather than on the course.
"It will be fun, nice to play with him," Koepka said. "We actually talked about it on Saturday when we played at home. I don't think we've ever actually been in contention together."
Rose was disappointed to bogey the par-five 18th in his 68, but was pleased with his overall performance as he continues to struggle with a shoulder problem.
"It's n othing major, just a tiny bit of tendonitis," the world number 13 said. "It's not impeding my golf.
"It was a disappointing way to finish. I felt like I was running out of steam a little bit coming in, but it's my f irst tournament in a month or so, so to back up a good start with another good round is good and now I feel like I'm into the weekend, which is where you want to be."