Tiger Woods saw off challenges from Padraig Harrington and Ross Fisher to take control of the US PGA Championship heading into the weekend at Hazeltine National Golf Club.
Woods sank three birdies in a row on the back nine of his second round but closed with a bogey en route to a two under par 70.
The World Number One now leads the field by four shots having once again won his duel with defending champion Harrington, who suffered three bogeys in a row on his back nine and also bogeyed the last for a one over 73.
England's Fisher had made it a three-way battle in the Minnesota heat but faded with a bogey, bogey finish for a 68 at three under alongside Ireland's Harrington, Fiji's two-time winner Vijay Singh (72), US Open Championship winner Lucas Glover (70) and Australian Brendan Jones (70).
England's Lee Westwood had got to four under after a 12 foot birdie putt at the 15th but a double bogey at the 17th sent him to a 72, two under for the tournament.
Woods has won all eight Majors he has led at the halfway stage and with a four shot lead the odds are shortening on a 15th Major victory of his career to move closer to Jack Nicklaus' record of 18.
Asked if he was now in a position to run away from the field over the final two rounds, Woods replied: "I don't know, there's a long way to go, 36 holes. I'll just keep plodding along.
"That's what I did today, I got off to a poor start but I just hung in there and that's what I'll do tomorrow and we'll see what happens. It was a tough day out there, the wind was blustery, the greens were bumpy and it was a little bumpy all round."
Woods had held a one shot overnight lead over Harrington having shot an opening five under par 67 at the 7,674 yard Minnesota course, with Singh a further stroke behind at three under in a group tied for third.
A morning level par round of 72 by the Fijian in favourable conditions left Woods still in charge as they began in the later wave of starters with the wind picking up considerably.
Woods and Harrington were trading blows for the third day in a row, the American having prevailed on the last day of the World Golf Championships-Bridgestone Invitational and the first day at Hazeltine.
Three-time Major winner Harrington, though, was quickly back on level terms on Friday as the four-time US PGA champion bogeyed the first.
Graeme McDowell and Alvaro Quiros both joined the lead at four under as Harrington dropped a shot, bogeying the first par five of the day, the 633 yard third, before rebounding with a birdie two at the fourth.
Both Northern Irishman McDowell and Spaniard Quiros faded while Woods moved back into the outright lead with a 15 foot birdie putt at the sixth.
Harrington birdied the ninth to close the lead on Woods, who then fell back into a tie with the Irishman at five under after leaving his approach shot at the tenth short on the lower tier of the green, from where he three putted.
England's Fisher had started well, getting to two under after he birdied three of his first four holes to go out in 33 and he added three more at the 13th, 14th and 16th to get to five under and into a tie with Woods, only to fall off with his poor finish.
"In some ways I'm disappointed, but overall delighted," was Fisher's verdict. "I felt like I hit the ball fantastically well today.
"At the same time, to finish bogey, bogey always leaves a little bit of a sour taste. But I'm still in there with a good shout with 36 to go."
Harrington's three bogeys in a row left him at two under but he staged a rally that began with a birdie at the 14th and when they got to the 642 yard par five 15th, the Irishman sent in a magnificent three wood approach shot from a fairway bunker 300 yards out to within 15 feet of the hole.
Woods was relentless, though, his birdie taking him to minus seven while Harrington's eagle putt agonisingly lipped out and his birdie kept him three shots back but in outright second place.
Then came the closing bogeys, Woods retaining his four shot lead in the process.
England's Ian Poulter fought his way to a 70, leaving him on two under at the halfway stage of the tournament, with Northern Ireland's Rory McIlroy on level par after a 73, McDowell and Quiros both falling away dramatically to finish at one over after a 75 and 76 respectively.
South African duo Ernie Els and Tim Clark matched Fisher with the day's low score of four under 68 among the early wave. Els moved to one under par, Clark to level par.