The 27-year-old Australian collapsed Friday on the ninth hole because of an attack of vertigo. He finished the round, and 36 holes into the 115th U.S. Open at Chambers Bay, he sat at two-under with a chance to win his first major.
On Saturday, Day collected six birdies, five on the back nine, to climb up the leaderboard and join Dustin Johnson, Jordan Spieth and Branden Grace for a view from the top.
Day's caddie, Colin Swatton, said his comeback on Saturday was "a superhuman effort."
"Greatest round of golf I've ever watched," Swatton said according to the Associated Press.
Spieth, who won his first major two months ago, began the day as the overall leader but he struggled to maintain that position with his worst round of the championship, a one-over 71 thanks to five bogeys.
Johnson kept his cool through the third round, finishing even at 70, even with a double-bogey on the 13th hole. Grace avoided any major mistakes, shooting even par and as the leaders' scores came down.
J.B. Holmes may have had the shot of the day at the 372 yard, par 4 16th hole.
After his drive ran off the green and into right greenside bunker. From 26 yards out, Holmes chipped the ball up past the hole, after rolling to the peak of the incline it came back down and dropped in the hole for an eagle. Holmes finished one-over 71 on Saturday and is tied for fifth at one-under.
Rory McIlroy quietly shot par on Saturday, but at four-over (tied for 25th), the No. 1 golfer in the world is essentially out of the running for his second U.S. Open win.
Day, who was brought to the ground by a vertigo attack, has collected four birdies on the back nine and is now tied for second with Branden Grace and Jordan Spieth at three-under for the championship.
Dustin Johnson held the lead for a few holes and even dipped down to seven-under for a brief moment before he recored a double-bogey on No. 13. Johnson still leads at four-under with five holes to play.
Patrick Reed shot himself out of the top spot on Saturday with three double-bogeys. He's six-over on the round -- one-over overall -- with five holes left. He's currently tied for 11th with Henrik Stenson, another former leader of the U.S. Open.
Spieth has five bogeys in the third round, which is one more than he had through the first two combined.
Dustin Johnson weathered a quick run from Jordan Spieth, held on until Patrick Reed hit two double-bogeys, and even kept it together while Branden Grace took at turn as the leader for a while Saturday at the U.S. Open.
Now, it's Johnson's turn. He's one-under through 11 holes, five-under overall, and has a chance for his best finish ever at the U.S. Open. He finished tied for fourth last year at Pinehurst.
But every hole is a rollercoaster at Chambers Bay.
Johnson was briefly alone at the top spot before he scored a bogey on the eleventh hole, dropping him into a tie with Grace. Those two are followed by Spieth (four-under) and Shane Lowry (two-under) at third and fourth overall.
All four are perfectly capable of scoring a few more birdies through their last holes. But they could also put up some bogey too. No lead is safe in University Place, Wash.
Meanwhile, at the U.S. Open a fire has broken out.
Not really, but nearby there appears to be a warehouse engulfed in flames, sending a thick cloud of black smoke into the sky near the site of the 115th U.S. Open.
On the course, which is unaffected, Brendan Grace is now the leader at Chambers Bay at six-under with Jordan Spieth and Dustin Johnson now tied for second through eight and seven holes, respectively.
Grace is two-under through eight holes after shooting two birdies in Saturday's round. He also had an eagle in both the first and second rounds.
Spieth followed his back-to-back birdies from earlier with back-to-back bogeys. A 16-foot birdie putt put him back in control, but he needed an extra shot on the seventh hole. Back and forth he goes.
Johnson maintained his position by saving par on No. 8 after an errant shot rolled off the course, down a hill. Each time it appeared to be coming to a stop it just kept going.
Eventually, he was granted a drop and was able to get the ball back onto the green and two-putt to par.
Jordan Spieth began pulling away from the field Saturday afternoon at Chambers Bay as soon as his third round began.
The Masters champion sank birdie putts of 38 and 41-feet on the second and third holes.
Reed didn't fare as well on No. 2.
He carded a double-bogey there after sending his tee shot into the rough on the left side of the fairway. He then found the frontside bunker on his next shot and after that, the rear one.
Reed is now three-under, tied for third with Joost Luiten of Netherlands (par through five holes) and Dustin Johnson (one-over through four).
Brendan Grace of South Africa is also shooting par through four holes which leaves him at four-under, and in second place behind Spieth.
Louis Oosthuizen is trying to stage a comeback at the U.S. Open.
Oosthuizen was nine-over par through the first 20 holes of the tournament, which included a first-round 77. Since then, the 2010 British Open champion has been nine under. Through 12 holes Saturday, he sat five strokes behind the leaders.
He almost moved to under par for the tournament on the ninth when he nearly made a hole-in-one. His 226-yard shot rolled across the hole and bounced off the flag. He then needed two putts to bridge the gap from the nine-foot ricochet.
But he hit a setback on 530-yard, par-4 11th, misjudging a six-foot putt for his first bogey on the day. He made up for it by sinking a birdie on the 12th hole.
Rory McIlroy's attempt to climb the leaderboard faded as he struggled to tame the greens on the back nine. He finished the round with an even-par 70 to remain at four over for the tournament.
Rory McIlroy's smooth start to the round hit turbulence on the 10th and 11th holes.
McIlroy, ranked No. 1 in the world, settled for par on 10 after missing a seemingly straightforward four-foot putt. He then bogeyed the par-5 11th after needing three putts to bridge the 46-foot gap from his approach shot. He stands at one under through 12 holes and three over for the tournament.
Louis Oosthuizen, who shot a first-round 77 playing alongside Tiger Woods and Rickie Fowler, is off to a strong start after his 66 on Friday. The South African is at three under through seven holes to move to within five strokes of leaders Jordan Spieth and Patrick Reed. Oosthuizen is nine under through his last 22 holes.
Of course, not everyone is finding cause for optimism on Chambers Bay's crispy links and greens. Sergio Garcia renewed his criticism of the Chambers Bay course after completing an even-par 70 to remain at five over.
"It's like playing [the] NBA Finals on a court with holes and slopes and no backboards," Garcia told Fox Sports.
Phil Mickelson is at four over on the day through seven holes and Angel Cabrera, the 2007 U.S. Open champion, moved to nine over with a round of 74.
Rory McIlroy is trying to climb back into contention.
The 2011 U.S. Open winner, who is targeting his fourth-career major title, moved to two-over par for the tournament after completing the front nine at Chambers Bay at two under. He's still seven shots behind of the leaders.
Phil Mickelson, however, continues to fade, bogeying two of his first three holes to move to five over. Mickelson has struggled since a strong opening round as he looks to complete a career Grand Slam by winning a U.S. Open title.
Former U.S. Open champion Webb Simpson fell further out of contention. He finished the round with a one-over-par 71 to move to six over for the tournament.
Jason Day, who collapsed on the course Friday after suffering from a case of vertigo, said he will play Saturday. Day finished the second round after being treated by medical workers on the final hole. He enters the third round tied for ninth at two under.
Jordan Spieth's dominant win at the Masters in April raised questions as to whether the 21-year-old wunderkind was poised to be the next great American golfer.
Through 36 holes at the U.S. Open, Spieth has lived up to his budding, yet bold reputation as one of golf's elite players.
Spieth shares the lead with Patrick Reed at five-under par heading into Saturday's third round at Chambers Bay Golf Course in University Place, Wash. Spieth vaulted to the top of the leaderboard Friday after finishing with a three-under 67 to tie for the day's second-best round. He made six birdies on the day.
Still, it won't be easy for Spieth to maintain his quest to win the second leg of golf's single-season Grand Slam.
Reed has put together two strong rounds, posting a 66 on Thursday and 69 on Friday to stay at the front. Just behind them are Branden Grace (-4) and Dustin Johnson (-4).
The third round of the U.S. Open started at 8:03 a.m. PDT. The leaders won't take the course until 2:50 p.m.
Be sure to check back here for live updates Saturday afternoon.