The final hour at Hazeltine changed everything.
Woods fired off three consecutive birdies, which began with a drive onto the fringe of the 299-yard 14th hole and ended with Woods nodding his head and dangling his tongue out of the side of his mouth after making a 20-footer.
He finished with 2-under 70 to build a four-shot lead, his largest through 36 holes at a major since the 2005 British Open at St. Andrews, when he led by five.
Woods is 8-0 when leading a major through 36 holes, and his chances only improved as everyone around him faltered.
"In order to have a lead in a major championship, you can't be playing poorly," said Woods, who was at 7-under 137. "All those times, I've been playing well. And I'm playing well now."
Ross Fisher was the last challenger to crumble. Tied for the lead with two holes to play, he came up woefully short on the par-3 17th and hit a drive that left him an awkward stance in the sand, both shots leading to bogey and 68. Padraig Harrington was tied with eight holes to play until he made three consecutive bogeys, closing with another bogey when he missed a 3½-foot putt. He finished with 73.
Before them was Graeme McDowell, who couldn't keep the ball dry and wasted a good start by shooting 75.
They all finished four shots behind at 3-under 141, along with Vijay Singh (72), U.S. Open champion Lucas Glover (70) and Brendan Jones (70), who played in the morning when the greens were at their smoothest, though still not easy.
Woods will play in the final group with Singh, one of many rivals in his career.
"There's a long way to go," Woods said.
Even so, it was the first time in three years that Woods opened a major with consecutive rounds under par.
Former Augusta State golfer Oliver Wilson is at 146. Augusta native Charles Howell (150) and former Louisville, Ga., resident Brian Gay (155) missed the cut.