The golf course is not a place usually associated with great acts of sporting courage.

The footy field, sure. The boxing ring, most definitely.

But on Saturday at Chambers Bay, Jason Day wrote a new chapter in Australian sporting folklore, one that will live long in the minds of those who saw it.

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A day earlier, he had collapsed on his final hole due to vertigo and his tournament appeared to be over.

But the Australian saddled up again on Saturday and, looking shaky on his feet, he emerged after 18 holes with a stunning 68 and a share of the US Open lead.

He birdied three of his last four holes, including the last two.

His caddie Colin Swatton said it was “superhuman” and the “greatest round of golf I’ve ever watched”.

“Every hole. Just feeding him water and giving him food, just giving him encouragement, just trying to get him through,” Swatton said.

“I said to him, ‘they’re going to make a movie about that round’.

“It was up there with Tiger Woods playing with a broken leg at the (2008) US Open. It was very impressive.”

Day withdrew from the AT&T Byron Nelson tournament three weeks ago after suffering from dizziness, blaming the spell on sleep deprivation.

But according to his agent Bud Martin, he was diagnosed with Benign Positional Vertigo after his collapse on Friday.

Day admitted to feeling groggy during the first half of his third round, but said he felt clearer as the round progressed.

Day tees off in the fourth round at 8am (AEST) Monday morning, on a course that nine-time major winner Gary Player described as one of the worst he has ever seen.

“I’d like to challenge anyone with a 16 handicap to do better than 40 over par,” Player told Yahoo Sports.

“It’s basically unplayable.”

A maiden major would be just reward for Day’s valour, although it certainly won’t be easy.

He is in a four-way tie for the lead with Americans Dustin Johnson and Jordan Spieth, and South African Branden Grace.