Golf: "The greatest round I've ever watched"

Jason Day, of Australia, watches his tee shot on the 14th hole during the second round of the U.S. Open golf tournament at Chambers Bay. Photo / AP.

Australia's Jason Day has defied his struggles with vertigo to surge to a share of the lead in the third round of the US Open at Chambers Bay.

Despite clearly battling the effects of the problem, world No.10 Day put together a brilliant two-under-par 68 on a brute of a course on Sunday to move to four-under for the championship.

"That was the greatest round I've ever watched," said Colin Swatton, his caddie and longtime coach who whispered words of encouragement along the hilly terrain of Chambers Bay. "I said, 'You've got the heart of a lion. You get to show the world today you get to be the greatest you can be and look, let's do it.' And he just put his head down and kept walking, one foot in front of the other. It was pretty impressive."

American Dustin Johnson was tied with him in the lead but he had four holes to play.

After collapsing on his final hole of his second round on Saturday and subsequently being diagnosed with benign positional vertigo, Day wasn't even a certainty to tee it up Sunday.

The 27-year-old was noticeably trying to keep his head level to prevent another episode - he struggled to tee his ball up and retrieve it from the cup all day.

Although his opening hole birdie try from 20-feet missed, Day was given a huge standing ovation by the grandstand off the first green, a sign of things to come as the masses rallied around.

A three-putt bogey on the second and another dropped shot after failing to get up and down from the sand on the fourth could have easily sent the Australian looking for the exit door but he knuckled down with five straight pars.

Then he made his move.

A precision approach to four feet on the 10th put him back under par for the week and right in the mix.

A bogey on the 11th was rubbed out with a birdie on the 12th and when he hit a shot inside 10 feet on the par three 15th and buried the putt it was game on.

Another long birdie on the 17th hole put him just one off the pace before he birdied the last.

Day has seven top-10 finishes in major championships, including three runner-ups, a third and a fourth.

Day's young countryman Cameron Smith, on debut in the majors, shot a brilliant 69 to move to one-under and a tie for fifth.

A top five finish would get him a US PGA Tour card.

Adam Scott shot a 72 to be tied 20th at three-over.

-AAP