Americans battle for lead as Day recovers from collapse

US Open contender Jason Day's continuing participation in the championship was in doubt on Friday night after the Australian collapsed on the course near the end of his second round.

Day, whose season has been affected by still unexplained dizzy spells despite brain scans, blood analysis and sleep tests, had been two shots off the then-lead of five under par as he walked towards his ball in a bunker at the ninth green, his final hole of the day.

Walking along the sloping fairway the 27-year-old dropped to the ground in front of a greenside grandstand and was complaining of vertigo as he was treated where he fell by medics who were quickly on the scene.

He returned to play after around five minutes on the floor, albeit clearly unstable on his feet as he walked into the bunker, taking a knee and a drink of water on the way.

Day then asked a rules official to remove a stone close to his ball under local rules as his hands were not steady before producing a great shot onto the green, the Australian two-putting for a remarkable bogey and a 70 to remain on two under before being led off the course by medics to a waiting cart.

He was made to sign for his round thus enabling him to continue play if fit for Saturday's second round and Day's agent Bud Martin later issued a statement promising further information.

“Jason is being treated on site at Chambers Bay, after feeling dizzy near the end of his round,” Martin said.

“We will provide more information later today after getting through the medical evaluation process. Jason wants to express his appreciation for all the good wishes from so many fans and friends.”

One of those friends was Tiger Woods who had been in the group behind Day, Jordan Spieth and Justin Rose and saw events unfold as he looked down onto the ninth green from the hole's elevated tee box.

“He was laying down there. And I know he didn't play in Dallas this year because of vertigo,” Woods said after his round of 76 which confirmed his early exit from the tournament with a missed cut at 16 over par.

“I played with him at Memorial and we talked about it in depth, and (he) did a blood panel and all that stuff. I hope he's okay. I'll call him as soon as I'm done here and see if he's all right.

“He's one of my really close friends. I'd like to get out of here and see if he's okay.”

Playing partners Spieth and Rose both said Day had seemed fine throughout a round which had seen the Australian hole out with a chip from 56 yards.

Rose initially thought Day had twisted his ankle on the difficult terrain and Spieth said: “It wasn't mentioned by him earlier in the round to me.

He may have mentioned it to (Day's caddie) Colin, but I was walking with him, 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 and keep the (spectator's) cameras off and let him just rebound from being dizzy.

"That's all it was, I think. I don't think it was a slip off of the ground. I think it was maybe a little dehydration or something. I'm not really sure. He didn't mention much after the round. We were trying to look out for him.”

Rose added: “At least he has a chance to recover for tomorrow, hopefully. I'm not sure if it's one of those things that's 10 minutes or a day, I don't know. Hopefully he'll be all right.”

Spieth still had an eight-foot birdie putt to attempt to put him back into the clubhouse lead and the Masters champion duly converted to post a three-under 67.

“That was one of the better birdies I've ever made given all the situation.... there was some wait time in between, but I actually got somewhat of a read off Jason's putt and was able to knock it in.”

Spieth's clubhouse lead was under threat out on the course as the afternoon wave of starters progressed their second rounds last night.

Overnight co-leader Dustin Johnson took his first-round finish on five under a shot further with a birdie at the uphill par-four seventh that has been statistically the most difficult hole on the course and that put him into a tie with fellow American Patrick Reed, two under for the day after birdies at the second and fourth.

South Africa's Branden Grace matched Spieth's 67 to move to four under, a shot ahead of a large chasing pack that included Johnson's first round co-leader Henrik Stenson, the Swede dropping shots at the 14th and 15th holes having started at the 10th.

Shane Lowry is nicely placed at one under following a level-par 70 but world number one Rory McIlroy's bid to move into contention was stalling, the 2011 US Open champion opening with a birdie only to follow up with bogeys at the third and seventh to drop back to three over par, some nine shots off the pace.

That also brought the projected cut line of five over uncomfortably into McIlroy's rear-view mirror and Graeme McDowell was even closer, one over approaching the turn and on the mark at five over.