"It's a good score," Johnson said. "I'm definitely happy with what I shot. I'm just not happy with the way I finished. Making two doubles, there's no excuse for that, especially the way I'm playing right now."
Johnson ran off five straight birdies to close out a 30 on the front nine of Sheshan International and a five-shot lead over Poulter. For his next trick, the 29-year-old American hit wedge four times from inside 100 yards before he could get the ball on the green at the 10th hole. He had to make a 12-foot putt for double bogey.
He followed with another run of four straight birdies, hitting a 5-iron into 15 feet for a two-putt birdie on the par-5 14th, and a 3-iron to the front of the 16th green for a chip-and-putt birdie that stretched his lead to six shots.
Johnson's tee shot sailed to the right and into the middle of the lake on the 18th. It appeared that he could have dropped further up the fairway, but, not wanting to take any chance, Johnson opted to return to the tee. He ripped another drive down the edge of the water, this time with his draw to reach the fairway.
But his approach went left near the lip of a bunker, and he did well to blast out to 15 feet and take two putts for his 7.
Fred Couples extended his lead to five strokes after the third round of the Champions Tour's season-ending Charles Schwab Cup Championship in San Francisco.
Winless this season, Couples shot a 3-under 68 to reach 15-under 198 at TPC Harding Park.
Mark O'Meara was second after a 67.
Dustin Johnson -18
Ian Poulter -15
Graeme McDowell -14
Graham DeLaet -12
Justin Rose -12
Rory McIlroy -12
Phil Mickelson -5