Three players took turn sharing the lead with Tiger Woods until he stood alone at the PGA Championship.

The final hour at Hazeltine changed everything.

Woods fired off three straight 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 a 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. "And all the times that I've been in this position, I have played 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 a 68. Padraig Harrington was tied with eight holes to play until he made three straight bogeys, closing with another bogey when he missed a 3 1/2-foot putt. He finished with a 73.

Before them was Graeme McDowell, who couldn't keep the ball dry and wasted a good start by shooting 75.

Fisher and Harrington 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.

"I mean, yes, Tiger is the greatest golf I think we've ever seen," Fisher said. "But at the end of the day, he's just like me and you. He's just a human being. He just happens to be damn good at golf. So we've got to work really, really hard to try and compete with him and catch him."

Woods has not lost a 36-hole lead on the PGA Tour in five years, dating to the Byron Nelson when he was revamping his swing. He came to Hazeltine having won his last two tournaments, however, and appears to be hitting his stride.

His finishing kick Friday was particularly impressive.

He avoided his second bogey in three holes with a 12-foot par save at No. 12, missed a birdie opportunity to take the lead on the 13th, then poured it on. The tees on the 352-yard 14th hole were moved up, giving players the option of trying to drive the green. At only 299 with the wind in their favor, it wasn't much of a choice.

Woods flushed his 3-wood onto the green and just onto the fringe, nearly holing the eagle putt. On the par-5 15th, he hit 3-wood through the green and chipped to tap-in range to widen the lead. Then came the 16th, and a putt that gave this major that inevitable feeling of Woods hoisting the Wanamaker Trophy for a record-tying fifth time.

"His game looked solid again today," Harrington said after playing with Woods for the third straight round, dating to Sunday at Firestone when Woods overcame a three-shot deficit to beat him. "I think he's in a good position. The reason he's a good front-runner is he can pick and choose his shots, and he's not been pushed into shots that he doesn't have to hit. And he's very good at that."

Harrington faced one of those shots, and it was a dandy.

Trying to get back into the game at the 15th, his ball on a slope in the bunker, Harrington thought the slope would help him reach the green, and he pounded a 3-wood that was perfect. It stopped 15 feet from the cup.

Woods called it one of the best shots he had ever seen, "worth the price of admission."

"He did say to me actually he would have paid to have seen it," Harrington said. "So I asked him for 50 bucks."

Woods gave nothing to anyone at Hazeltine -- not even himself.

"There's a long way to go," he said.

Still, it was the first time Woods has opened a major with consecutive rounds under par since the 2006 PGA at Medinah.

He will be paired in the final group Saturday with Singh, one of his many rivals. They have not played together since the opening two rounds at the Deutsche Bank Classic in 2007, and not in a major since the first two rounds of the 2004 PGA Championship at Whistling Straits, which Singh won for his third major.

"Tomorrow, you position yourself," Woods said. "You have to make sure you're there and in position, and I know Vijay isn't going to make a lot of mistakes. He doesn't. He's going to be very consistent."

A dozen players from nine countries remained under par going into the weekend, five of them major champions. The group included Ernie Els, who rebounded with a 68 despite again missing a half-dozen birdie chances inside 8 feet.

Woods built his lead with birdies at the end. Equally important were pars throughout his round.

He made back-to-back 6-foot par putts, the latter after hitting into the bunker on the par-3 fourth, and regained control with a birdie from 20 feet on the sixth and a chip to 4 feet for birdie on the par-5 seventh.

Then came the struggle. After a three-putt bogey from 70 feet on the 10th, he had to scramble for par on the 11th by making a tricky 4-footer, then followed that by going from the right rough through the 12th green, an average chip and a 12-foot par that felt just as good as some of his birdies.

"I could have easily shot a couple over par," he said. "But I turned it into an under par round."

Not so for Phil Mickelson, playing his first major since the U.S. Open while missing most of the summer as his wife and mother battle breast cancer. Lefty again struggled on the greens, particularly the short putts that could have kept his round going.

He wound up with a 74 for the second straight day, the first time he has failed to shoot par or better in the opening two rounds of the PGA Championship. Only late in the day, as the greens turned bumpy, did he make the cut on the number at 4-over 148.

"I'm not going to beat many people putting the way I am," Mickelson said. "I've got to get this thing turned around."

For Woods, he again is headed in the right direction.