UNIVERSITY PLACE, Wash. — The 115th United States Open had the wind briefly knocked out of it Friday afternoon when one of the leaders, Jason Day, fell in a heap on the ninth green. Day, sitting two strokes behind the pacesetters after a birdie at the eighth, was walking toward his ball, which had landed in a back bunker.  One second, Day was upright. The next second, he was not. Normally, the sight of a man down on a golf course would elicit swift reaction. But at Chambers Bay, built on an old sand and gravel quarry, the tumbles have not been restricted to players dropping down the leaderboard.  If nobody was quick to act at first, it was because already this week, the caddie of the Scot Stephen Gallacher had been sidelined when he slipped on the slick fescue grass. Henrik Stenson’s bagman, Gareth Lord, was working despite a cast on his left hand after taking a nasty fall on the 16th tee in a practice round. And earlier in the round, Tiger Woods, the 14-time major champion who was playing in the threesome directly behind Day, lost his footing and landed on his backside, adding indignity to his ignominious 36-hole score of 16 over.  Day fell on his side and rolled onto his back, and his hand flew to his face. Day’s caddie and coach, Colin Swatton, was the first to arrive at his side. Day missed time last year with wrist and back injuries. The fear, at first, was that he had slipped on the side slope and reinjured his back or hurt his wrist trying to break his fall. The reality, in a way, was scarier.  Day told the first two emergency workers who arrived at the scene that he had experienced another episode of the vertigo that forced him to withdraw from the Byron Nelson tournament in Dallas last month. Earlier in the week, Day said he recently had submitted to three sleep studies, undergone extensive blood work and had a magnetic resonance imaging test done on his head and neck to try to pinpoint the cause of tingling in his arms and “the shakes,” as he described it.  All the results came back negative, Day said, leading his doctors to conclude that he was suffering from sleep deprivation and dehydration. Day had been getting plenty of sleep this week, making this episode unsettling.  Day, a 27-year-old Australian with three top-four finishes in the past four Opens, was playing in a threesome with Jordan Spieth and Justin Rose. As the paramedics gathered around Day, Spieth played traffic cop, shooing away photographers and asking the television camera operators to stop taping.  Two more pairs of emergency workers ran onto the green, and they were joined by Day’s manager, Bud Martin, and Day’s wife, Ellie, who is pregnant with the couple’s second child. Ellie leaned over and asked Day if he had been feeling lightheaded before he began the walk from the elevated ninth tee to the green, a drop of 105 feet. She said he told her that he had not, that he felt fine one moment and dizzy the next.  Day lay on his back for roughly five minutes while his blood pressure was taken. He was helped to his feet, and as he rose, the fans in the grandstand behind him applauded. Day walked unsteadily to the bunker, blinking unceasingly and shaking his head as if he were seeing more than one in the sand.  Once in the sand, he tried to remove a pebble that was behind his ball, as allowed by the rules, but was afraid of accidentally moving his ball because his hand was shaking. The rules official walking with the group stepped in and moved the impediment for him. Day hit out of the bunker, and his caddie helped him out. He knelt and leaned on his club while Rose lined up and missed a 24-foot birdie putt.  Day two-putted for a bogey 4, kneeling with his head down between putts, for an even-par 70. As Spieth studied his eight-foot birdie putt, Day walked off the green, with the help of emergency workers, and was whisked in a cart to the scoring trailer to sign his scorecard. Day was taken away in a Pierce County Sheriff’s Department van to the players’ locker room. An unbroken line of fans clapped and yelled well-wishes at Day as the vehicle disappeared down an asphalt path.  Spieth, who drained his birdie for a three-under 67 that left him tied for the early lead at five-under 135, walked off the green looking awfully serious for someone who had placed himself in contention at the halfway point of his national championship.  “I was walking with him,” Spieth said, referring to Day, “and the next thing I know, I turned around and I think he got dizzy and slipped and fell. So at that point, how can we help him out and kind of clear the scene and try to keep the cameras off and let him just rebound?”  There is precedent for a player recovering from a scary on-course episode in the second round of a tournament to end up in the winner’s circle two days later. In 2012 outside Orlando, Charlie Beljan staggered through 18 holes with a racing pulse caused by a panic attack. After signing for a 64, he was driven to the hospital in an ambulance. Upon being released the next morning, Beljan finished the tournament and held on for his first PGA Tour victory.  Beljan teed off three groups ahead of Day. Woods, who is good friends with Day, was standing on the ninth tee when play came to an eerie standstill. “I know he was laying down there,” Woods said, adding, “I hope he’s O.K. I’ll call him as soon as I’m done here and see if he’s all right.”  In a statement, Day’s manager, Martin, said that doctors told Day he had benign positional vertigo and that Day was hopeful he would be able to compete this weekend. Martin said Day was resting comfortably after being treated on the grounds.  The 115th United States Open continued, but the events of the day had left everybody feeling a little wobbly.  A version of this article appears in print on June 20, 2015, on page D1 of the New York edition with the headline: Mood Turns Serious After Health Scare. Order Reprints| Today's Paper|Subscribe
Stumbles have been a common sight on the slick surface at Chambers Bay this week, but Day’s episode of vertigo was a much scarier moment.