Woods - takes three-shot lead into Saturday. (Allsport)

Only Ireland's Padraig Harrington could stop Tiger Woods from running clean away with the United States Open at rain-sodden Bethpage Black on Friday.

Those who believe that Woods walks on water had no reason to change their minds as he added a 68 to his opening 67 for a five under par halfway total of 135.

For most of the day his clubhouse lead was an incredible eight strokes, but then came Harrington with the performance of his season so far.

Despite conditions so bad that parts of the course - on New York's Long Island - were flooded, the Dubliner reached the turn in 35 to stay level par, then birdied the long fourth and 408-yard sixth.

By parring the last three holes, including the seventh from a greenside bunker and the short eighth from wide of the green, Harrington returned a 68 as well and limited Woods' overnight advantage to three strokes.

He was himself four clear of those in joint third place on two over.

That group comprised Korean KJ Choi, Jeff Maggert, Davis Love and Sergio Garcia, whose 74 was a real rollercoaster ride.

But the day, like the first, had to belong to Woods, who with victory on Sunday will be halfway towards an unprecedented Grand Slam of all four majors in a season.

He had some more inspired putting to thank for his score again. He took 28 putts in the first round and shaved one off that in the second.

"I don't know how much more rain the course can take," said Woods, who described himself "ecstatic" about the position he was in.

Even though it was pouring throughout his morning round, he appeared to have got the better end of the draw. But how he made the most of it.

While others were humbled by the longest course in the championship's history - 16-year-old qualifier Derek Tolan had an 80 and Filipino Felix Casas a 92 - Woods took a mighty leap towards his eighth major title.

Nick Faldo, himself going back from level par to six over with a 76, had joked before the championship that he would quit if Woods matched his record 12-under performance at Pebble Beach two years ago.

Incredibly, the world No 1 was halfway towards that mark after just 22 of the 72 holes.

Woods' opening birdie was truly inspired. He pushed his drive into rough and had to negotiate the branches of a tree as well for his second shot, but from around 150 yards he put it to three feet.

The 26-year-old Masters champion, winner of six of the last 10 majors and seven in all, then pitched to 18 inches on the second and chipped to six feet at the 517-yard fourth.

He failed to get up and down from sand on the short eighth, had to hole from 18 feet for his par at the next after going from rough into another bunker - that gave him an outward 33 - then bogeyed the 492-yard 10th after being in a greenside trap again.

But Woods simply refused to lose his focus. Seven pars followed, some of them from unlikely positions, and when he had an 18-foot chance to birdie the 411-yard last, one somehow sensed he would make it. He did.

Garcia, second overnight only one behind, double-bogeyed the 435-yard 11th - his second - after going from right rough to left rough to rough over the green, but limited the damage after that.

The young Spaniard's round was not without its controversial moments, though.

He appeared to take offence at a remark from the crowd during his back nine, backing off a shot and gesturing.

The comment may well have had something to do with his habit of re-gripping the club time and time again before he feels ready to hit.

Then after bogeying the eighth - his 17th - he displayed his anger again by slamming the ball into the sodden green.

Darren Clarke, paired with Woods, had his second successive 74 to be eight over, but that was safely inside the projected cut mark.

Faldo, joint seventh overnight, double-bogeyed the first after hooking his drive into much worse rough than Woods had found.

The 44-year-old, playing on a special invitation that came only two weeks ago, could chop it forward only 60 yards, then spun his pitch off the front of the green and three-putted.

A 76 was probably not a bad effort after that - many others would have taken it - but he was 11 adrift.

Dubliner Paul McGinley crashed to a 79 for 14 over and Dane Thomas Bjorn, one over at the start, also scored 79 to be 10 over and borderline for qualifying.

Open champion David Duval, for a long while the only player apart from Woods under par for the day, double-bogeyed the seventh (his 16th) and triple-bogeyed the short eighth to finish 11 over.

A second successive missed cut in the majors this year loomed for Duval, while defending champion Retief Goosen knew he was out.

Goosen, who slumped to a 79 on Thursday, also took six at the eighth for a 75 and 14 over aggregate.

Colin Montgomerie, five over when he teed off again, was fighting to stay in the event after a a triple bogey seven at the sixth.

With two to play he was 10 over - the likely cut-off mark. Paul Lawrie finished six over after a second successive 73.

But he could only par it and with a 76 for 11 over faced the probability of his first missed cut in 11 trips to the event.

Harrington, fifth in the Masters, said: ``It was a good day to play well and I was not getting distracted by the weather.

``Over the years guys who have been paired with Tiger on Saturday or Sunday have tended to be affected negatively.

``There can be a distraction factor, but I've played with him and I'm not interested in what he is going to do, only what I do.''

Montgomerie did not have to wait long for confirmation that he was out along with Per-Ulrik Johansson, McGinley, Goosen and David Howell.