The world No. 1 followed his opening 68 with the low round of the day to move to 13-under 131 heading into the weekend. Johnson birdied the last four holes for a back-nine 29 at Sheshan International Golf Club, the same course where he won in 2013.
"Obviously I played very well today, but I actually got a putter about 20 minutes before I teed off," Johnson said, "It's the (TaylorMade) Spider, so it's one that I've used before, but I wasn't really pleased with the one I played with yesterday.
"The guys here in China made me one, and I got it right before I walked to the tee, hit a few putts on the practice green with it and I was like 'Oh, this will work pretty well.' Then I went out and holed a lot of putts, so, I kind of like it.
"It's a golf course that I enjoy playing. It suits my eye very well. It's still, though, you've got to drive it well. For me, if I drive it good out here, I feel like I can make a lot of birdies. The par 5s, I can reach all the par 5s if I drive it in the fairway. You know, so those are where I've got to attack those. The rest of the holes, I feel like if I'm in the fairway, I can attack the golf course."
Johnson and Koepka are close friends who live down the street from each other in Florida.
"Yeah, hopefully. It would be great," Johnson said of the pairing with Koepka. "He's one of my best buddies. It would be a lot of fun out there playing with him. We live right down the street from each other, so we hang out a bunch. It's good, going to the gym, having somebody to work out with, pushing each other in the gym helps a lot.
"You know, it's funny, we don't really play much golf, though, at home together. We go to the gym and we hang out but we don't go to the golf course together. I think we played last Saturday; that was the first time we played in, I mean, I couldn't tell you how long. All year, probably."
Koepka, the reigning U.S. Open champion who led after the first round with a 64, followed with a 68.
"I felt like I played pretty well. I felt like I drove it really well," Koepka said. "That's what you've got to do out here to hit fairways. I felt like we were hitting quite a bit.
"Then a couple of the putts, we missed some short ones, but to be honest with you, I felt like I hit them where I wanted to. I think I misread maybe two of them, especially the one at the last. It kind of changed at the last second over the ball. But you know, that's going to happen over the course of 72 holes. But feel like I'm playing well and just need to keep hitting fairways out here.
"To be honest with you, I didn't care what Dustin is doing. I'm kind of focused on what I've got to do. Yeah, it did get kind of boring out there for awhile, 13 straight pars. I felt like I hit some good putts that didn't go in. I had some good opportunities -- too, and just didn't convert them. I wasn't really focused on Dustin. If I take care of business and keep doing what I'm doing, I should be right up at the top, which I am. I just need to play good this weekend."
Justin Rose of England shot a 68 to get to 9 under and will join Johnson and Koepka as former U.S. Open champions in the final group Saturday.
Kiradech Aphibarnrat of Thailand carded a 70 and joined Rose in a tie for third place. American Patrick (70) was alone in fifth at 8 under.