UNIVERSITY PLACE, Wash. - On Friday afternoon, Jason Day was lying by the side of the ninth green with paramedics hovering over him after he suffered an attack of vertigo. He managed to finish that hole, his last of the round, on unsteady legs, but there was some question whether he would return for the third round.

Day not only returned but -- even as he clearly continued to suffer the effects of dizziness every time he bent over to tee up his ball or pick it out of the cup -- scaled the heights to the top of the U.S. Open leaderboard. He made five birdies on the back nine, including three of the last four holes, to shoot a 2-under 68 for a 54-hole total of 4-under 206 Saturday at Chambers Bay.

When he came off the course, the gutty Australian was tied for the lead with Dustin Johnson (70) and South African Branden Grace (70). Second-round co-leader Jordan Spieth, who was paired with Day the first two rounds and witnessed his second-round collapse, shot 71 to join them in a four-way tie at the top.

Because of his condition, Day answered two quick media questions after his round before leaving. "I didn't feel that great coming out early," he said. "I felt pretty groggy on the front nine just from the drugs that I had in my system, then kind of flushed that out on the back nine.

"But then the vertigo came back a little bit on the 13th tee box, and then I felt nauseous all day. I started shaking on the No. 16 tee box and then just tried to get it in, really. Just wanted to get it in."

Day withdrew after two holes of the third round of the WGC-Bridgestone Invitational last year after a bout of vertigo and didn't play in the AT...T Byron Nelson last month because of the condition. Caddie Colin Swatton said Day considered withdrawing three times Saturday.

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Describing his current condition as "worse" than the attack that forced him to withdraw at Bridgestone, Day added, "I think the goal was just to go through today and see how it goes."

After a week of moaning about the condition of the greens and the course setup at the first-time major championship venue, Day showed that fortitude might be as important as shot-making in determining a champion.

Another who matched Day in terms of will and determination was South African Louis Oosthuizen, who was in a four-way tie for fifth at 1-under.

On Thursday evening, Oosthuizen was part of the pity party with Tiger Woods and Rickie Fowler, who all laughed and consoled each other for their miserable opening rounds in the U.S. Open. Oosthuizen's 77 wasn't as bad as Woods' 80 or Fowler's 81, but he was tied for 135th. After bogeys on his first two holes in the second round, he had one foot on the flight home.

Then something clicked. Oosthuizen made a U-turn to shoot a pair of 66s, which ranked as the best scores in the second and third rounds. While the leaders battled extremely difficult afternoon conditions, Oosthuizen sat back and watched his name move up the leader board.

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"Being 9 over through 20 holes, it looked like I would have been back in Florida today," he said. "But I never gave up. Even if you're playing badly, you want to work on something to take it into the next week. I started hitting the ball better and better in the second round and hit it really well today.

"It just shows you to never give up, especially on a golf course like this. If you play well and shoot 1 or 2 under, you can really climb the leader board."