One birdie on the back nine was enough for a 1-under 71. It gave him a two-shot lead over three-time major champion Padraig Harrington and Y.E. Yang.
Woods has never lost a major when leading going into the final round.
Only once in his career has he lost any tournament when leading by two shots or more.
"I played conservatively today," Woods said. "I didn't give myself a lot of looks. I was lag putting a lot. The only putt I really missed was on the three-putt. Other than that, it was a good, solid day.
"Given the conditions and my position in the tournament, I didn't mind it."
He was at 8-under 208, finishing just as the rain arrived in Minnesota.
Harrington surged into a share of the lead with four birdies over an eight-hole stretch in the middle of the round, catching Woods with a 7-foot birdie putt on the short par-4 14th. Right when it appeared they would be paired in the final round for the second straight week, Harrington made his only bogey of the round by going over the 18th green and failing to save par.
He wound up with a 69, and much greater hopes of defending his PGA title than he had starting the day.
"The narrower the gap, the better," Harrington said. "If I have to take four shots and I've taken two the first day, I suppose we're halfway there. Obviously, to get a win, you've got to beat him by three tomorrow. That's a tall order. But as I said, everybody in the situation who is behind is going to think, 'Well, we have nothing to lose.' You've got to have that attitude."
Woods will play in the final group with Yang, who matched the best round of the tournament with a 67. Yang won his first PGA Tour event earlier this year at the Honda Classic, although the 37-year-old from South Korea is better known for taking down Woods at the HSBC Champions in China three years ago.
They weren't playing in the same group in 2006, however. And this will be Yang's first time contending in a major.
"It will be my first time playing with him, so I'll try not to go over par," he said with a smile. "But I've been looking forward to it. I've thought about playing with Tiger recently. Surprised it came true so fast."
Woods' four-shot lead was his largest in a major after 36 holes since he led by five at St. Andrews in 2005. Just like that British Open, his margin was cut to two shots going into the final round. Suddenly, there are other challengers to try to stop Woods from winning his 15th career major, and first of the year.
Henrik Stenson, who captured The Players Championship in May, had a 68 and was in the group at 4-under 212 along with U.S. Open champion Lucas Glover (71).
Ernie Els pulled within one shot of the lead until he finished with three straight bogeys, leaving him with a 70 and five shots behind. He was disgusted with the end of his round, although the Big Easy spoke for so many others about the outlook Sunday. Woods has never been beaten at a major when leading. But at least they have a chance.
"You could really feel that there's a real championship going on around you," Els said. "It's not a runaway deal. Looked like a runaway thing at the end of yesterday. But it looks like the guys are really set to give Tiger a go, and the crowd could sense that."
Woods, however, has a major advantage.
He has never lost in America when leading by more than one shot, and the only time anyone beat him from that position was Ed Fiori at the 1996 Quad City Classic, when Woods was a 20-year-old playing his third event as a pro.
Lee Westwood came from two shots behind to beat Woods in the Deutsche Bank-SAP Open in Germany in 2000.
Woods appeared to be on his way when he stuffed a short iron into 4 feet for birdie on the second hole. Then came a three-putt bogey on the par-3 fourth, which he left woefully short. He missed the fairway on three par 5s, which he could have reached in two from the fairway. Instead, he was aiming away from trouble, not willing to give away shots.
On this day, everyone was taking their best shot. "I thought it was going to be playing a little bit more difficult today, but it wasn't," Woods said. "I just felt that with my lead, I erred on the side of caution most of the time. If I did have a good look at it, I took aim right at it. Otherwise, I was just dumping the ball on the green and two-putting."
The lead shrank quickly.
Glover pulled within two shots until he was slowed by a poor bunker shot on No. 10.
Harrington made his second straight birdie with a 20-foot putt on the par-3 eighth, rolled in a 6-foot birdie on the 11th and made a few solid par saves along the way to stay close to Woods. He caught him at the 14th, then tried to get to the clubhouse without any damage. He almost made it, but caught a flyer out of the rough and over the 18th green.
Woods wasted opportunities, and appeared to really blow a good chance when he drove just through the par-4 14th. His chip came out hot and through the green, against the collar. Unable to hit a proper chip or a putt, he used the blade of his sand wedge to roll the ball some 15 feet to the cup. It came out perfectly, and Woods showed fierce emotion when it fell for birdie.
Over his last hour, however, he couldn't wait to get off the course - a terrible pitch at the 15th some 40 feet short, a 7-iron he pulled over the green and close to a TV tower, his hand flying off the club on the 17th.
Still, he wound up with the lead, and Woods wasn't about to trade that position with anyone.