UNIVERSITY PLACE (Washington) - A day after collapsing on his final hole after suffering a bout of vertigo, there were doubts whether Jason Day would even tee off in the third round at Chambers Bay.

Now, the only question left is if the 2011 and 2013 US Open runner-up can go one step further in the tournament's 115th edition this morning (Singapore time).

Whatever happens, he has already delivered one of the gutsiest displays seen in Major history.

He battled dizzy spells, nausea and the shakes on Saturday to card a two-under 68 that put him in a four-way deadlock with co-leaders Dustin Johnson, Branden Grace and Jordan Spieth at four-under 206.

Just eight players were still under par by the time the dust had settled and only one player left in the 75-man field, Louis Oosthuizen (66), carded a better third round than Day.

How the Australian managed a back nine that featured five birdies, including three over his four closing holes, was arguably mind-boggling.

At times he struggled to stay on his feet. Before almost every shot, he was rubbing his temples. He had difficulty glancing up to follow his ball flight. Bending over to get the ball out of the hole was a challenge too.

But somehow he kept his game together as others around him came to grief on a golf course that takes no prisoners.

Evoking memories of Ken Venturi's 1964 US Open victory when he battled heat exhaustion to win the title, Day's efforts earned him a rousing ovation from the 6,000 fans packed into the 18th-hole grandstands.

The Australian took only a moment to savour the applause as he gingerly made his way to a waiting van, where he slumped into the back seat with closed eyes and laid his head on the back rest.

"Last year I didn't play the round after I had vertigo and this one was worse," the 27-year-old told reporters. "I think the goal was just to go through today and see how it goes."

On the front nine, he "felt pretty groggy" from the medicine he had been given to treat the vertigo. But the drugs did not prevent the vertigo from coming back on the 13th tee box, and Day was shaking on the 16th tee box.

His caddie Colin Swatton said he was in awe of the lion-hearted Australian's performance.

"I said to him on No. 18 that was the greatest round of golf I've ever watched," Swatton told The Seattle Times. "It was a superhuman effort."

Day was soon joined at the top by Johnson, who had a 70, Spieth with a 71 and Grace, who signed for a 70.

They were three shots clear of the field with four players tied on one under - Oosthuizen of South Africa, Cameron Smith of Australia (69), Shane Lowry of Ireland (70) and J.B. Holmes of the United States (71).

World No. 1 Rory McIlroy hit an array of superb shots, but at the end of the day his 70 meant he was still four over for the tournament. "I missed seven good chances on the back nine, or seven makeable putts anyway," said the Northern Irishman. "I feel like I turned a 65 into a 70 today. Just real disappointed."

Phil Mickelson's chances of finally winning a US Open, after a record six runner-up finishes, were wrecked when a 77 left him a distant 10 over.

REUTERS, AGENCE FRANCE-PRESSE