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The Australian, who collapsed on Friday after feeling dizzy, shot a two-under 68 to move to four under despite being "groggy" during his penultimate round.

Day leads with American pair Jordan Spieth (71) and Dustin Johnson (70), and South Africa's Branden Grace (70).

Louis Oosthuizen's 66 lifted him to one under with Ireland's Shane Lowry (70), while Rory McIlroy (70) is four over.

Day, twice a US Open runner-up, delighted the crowds at the course in the state of Washington on the USA's west coast, with a stunning back nine to take a share of the lead having been seven adrift of the leaders following a pair of bogeys on the front nine.

"I felt pretty groggy on the front nine from all the drugs," said the 27-year-old after his round. "I flushed that out on the back nine. The vertigo came back on the 13th tee and I felt it come back again on the 16th tee. My goal was just to get through and see how it goes."

Despite visibly struggling, the world number 10 rattled in five birdies on the back nine, including a birdie-birdie finish to shoot up the leaderboard and set the clubhouse lead.

The men tied with Day all led at various points of an incredible third round. Spieth, who was joint overnight leader with Patrick Reed (see 'the bad' below) was three clear after three holes but four bogeys around a birdie in his next six holes brought him back to the pack.

"Four three-putts, two of them I could not do much about," said the 21-year-old Masters champion. "I just need to limit the mistakes on Sunday."

Johnson had three birdies in five holes to reach six under after nine but a double bogey on 13 halted him. "I've been in the situation a few times so I know how to handle myself," said the 30-year-old, who led by three in 2010 but collapsed to an 82 in the final round. "I know what it takes to get it done."

Grace also reached six under, with a birdie on the eighth, but then dropped away with three bogeys in five holes before getting back to four under with a two on the par-three 15th.

"I'm stoked. I can't wait [for Sunday]," said the 27-year-old who has six wins on the European Tour but has only once finished in the top 20 at a major.

All three had birdie putts on the last to take the outright lead but missed them.

Oosthuizen rewrote the record books on Saturday to move three shots off the lead.

When he bogeyed the opening two holes of Friday's second round, the South African had been nine over, playing alongside Tiger Woods and Rickie Fowler who finished a combined 30 over par.

But while the Americans went backwards, 2010 Open champion Oosthuizen made a birdie at his third hole then had four more and an eagle in a 66. On Saturday, five birdies and a bogey matched Friday's score as he continued his progression up the leaderboard to one under.

His total of 132 shots for the middle two rounds is a US Open record.

World number one McIlroy insisted he could still win this year's title despite remaining four over par after a level-par 70. The Northern Irishman, who won the 2011 US Open, picked up two shots on the front nine but missed seven makeable birdie chances on his way home as he dropped two shots.

He said: "I turned a 65 into a 70. I need something spectacular on Sunday. If I can play as well tee to green again and have one of my best ever putting rounds I have a chance."

Ireland's Shane Lowry is much better placed on one under after also carding a 70. Lowry reached three under on the 10th but bogeys on the 14th and 16th halted his momentum.

The undulating nature of the greens and fairways led many to believe that Phil Mickelson would have the imagination to conjure up a first US Open win to complete his career Grand Slam.

But the 45-year-old six-time runner-up had nine bogeys in a seven-over-par 77 to drop to 10 over.

The majority are keeping McIlroy company on four over. Englishman Ian Poulter had the best day with a one-under 69, and lies alongside compatriots Justin Rose (72), and Paul Casey (73), and Scotland'sMarc Warren (72).

Another Scot, Colin Montgomerie (72), and England's Luke Donald(73) are seven over while Lee Westwood hit a 77 to drop to nine over.

Marc Warren's birdie on the par-three 17th. The Scot's tee shot hopped down the steps into a greenside bunker and got caught up under a rake. He was awarded a free drop which plugged in the sand but he chipped in for a two.

Patrick Reed's third round at Chambers Bay is one he will be looking to erase from his memory. A six-over 76 saw the halfway joint leader drop away to one over. Three birdies cancelled out three bogeys but three double bogeys did the damage.

At least he wasn't Ben Martin. The American was two shots off the lead at halfway after rounds of 67 and 70. On 'moving day', he started with a birdie to get to within one of the lead on four under. Then the wheels came off. Four bogeys, three double bogeys, one triple bogey and one quadruple bogey added up to a 16-over-par 86 and 13 over total.

Day's caddie Colin Swatton called his player's round a "superhuman effort" and the "greatest round of golf I've ever watched".

South Africa's 1965 US Open champion Gary Player, 79, on Chambers Bay: "This has been the most unpleasant golf tournament I've seen in my life. The man who designed this golf course had to have one leg shorter than the other. This is a public course where we are trying to encourage people to come and play and get more playing the game. They are having a putt from 20 feet and they are allowing 20 feet right and 20 feet left."

USGA executive director Mike Davis, the man responsible for the Chambers Bay set-up: "It wouldn't be a US Open if there wasn't a little muttering". He added the putting surfaces were "better than they look".

Rory McIlroy, by his own admission, missed "seven makeable putts" in a level-par 70 to stay four over, then said of the greens, described as "like broccoli" by Swede Henrik Stenson on Friday: "I don't think they're as green as broccoli. I think they're more like cauliflower."

Sergio Garcia has moaned every day so it would be remiss to ignore his post round-three rant: "To me it's like playing the NBA finals on a court that has holes and slopes and no backboard. It just doesn't feel right.''

Moving man Louis Oosthuizen on his incredible last 34 holes since dropping to nine over after two holes on Friday: "Being nine over through 20 holes, it looked like I would have been back in Florida today. But I made a few putts, started hitting the ball well and could have been probably a lot lower."

Jordan Spieth on his chances of becoming the first man since Tiger Woods in 2002 to win the first two majors of the year: "I think as I sleep on it and wake up there will certainly be some nerves. It's not like I'm a veteran at this by any means."

- BBC