Day suffered an attack of vertigo on the final hole of his second round, but he was passed fit to tee up on Saturday and he fired a commendable two-under 68 despite feeling unwell throughout most of the back-nine and struggling just to pick his ball out of the hole.
But the Australian managed five birdies on the inward half, including three over the last four holes as he finished the day tied for the lead on four under with Jordan Spieth, Dustin Johnson and Branden Grace.
Day was seven shots off the lead after turning at two over for the day, and his caddie-coach Colin Swatton later revealed that he had considered quitting three times on the back nine as the medication he had taken to combat the vertigo began to wear off.
“I didn’t feel that great coming out early, 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,” said Day in a brief interview before he was taken back to his on-site motor-home to rest.
“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 16 tee box and then just tried to??get it in, really.
“Last year I didn’t play the round after I had vertigo (at the Bridgestone??Invitational) and this one was worse. I think the goal was just to go through??today and see how it goes.”
Swatton described Day’s performance as??”a superhuman effort” and the “greatest round of golf I’ve ever watched”, and the world No 10 will go out in Sunday’s final group with Johnson with Grace and Spieth in the penultimate group.
Spieth, bidding to become only the sixth man in history to win the Masters and U.S. Open in the same year, looked set to take control of the tournament when he holed monster putts for birdie at the second and third to move to seven under.
But his prowess with the putter deserted him as he bogeyed four of the next six holes, although he did claw a shot back with another birdie at the sixth. Spieth leaked another stroke at the 11th, but he steadied himself down the stretch and got back to four under after converting a superb tee-shot to the 15th before parring in to complete a 71.
Johnson bogeyed the second and fourth either side of a birdie at three, and three birdies over the next five holes lifted him to six under. But after a birdie at 12 offset another dropped shot at the 11th, Johnson made a mess of the next and ran up a double-bogey six.
The American parred the last five to remain at four under, while Grace earned a share of the 54-hole lead at a major for the first time in his career despite dropping three shots in five holes around the turn having birdies five and eight to claim the outright lead on six under.
The South African sent a pinpoint tee-shot to four feet at the 252-yard par-three 15th and rolled in the putt, and three solid pars coming in capped a creditable 70.
The leading quartet have a three-shot advantage over a resurgent Louis Oosthuizen, who broke a U.S. Open record with his second consecutive 66 that hoisted him to one under.
Oosthuizen, who was nine over after two holes of his second round, carded five birdies and one bogey to become the first man to shoot 132 over the middle two rounds of the tournament.
The 2010 Open champion was joined on one under by Ireland’s Shane Lowry, who got to three under after his third birdie in five holes at the 10th before struggling coming in – dropping shots at the 14th and 16th to return a 70.
Australian Cameron Smith (69) and JB Holmes (71) are the only other players under par for the week, while overnight joint-leader Patrick Reed plummeted to one over after a torrid 76 which included three double-bogeys.
Rory McIlroy wasted several chances to get into contention for the final round, again failing to back up his superb long game with a putting display to match as a disappointing two-birdie, two-bogey 70 left him eight shots off the lead on four over.