Day was walking from the elevated tee on the par-three ninth – his final hole – at Chambers Bay when he suddenly fell to the floor.
The world number 10 was quickly attended to by paramedics and was heard on television saying that the incident was related to “vertigo I’ve had for a while”.
After several minutes the 27-year-old was able to get unsteadily to his feet and complete the hole, although he was unable to get up and down from a greenside bunker and had to settle for a round of 70 to finish two under par.
Day withdrew before the start of the Byron Nelson Championship three weeks ago due to severe dizziness and revealed on Tuesday he had undergone numerous tests to try to identify the cause.
“I had three sleep studies done. I had a lot of blood tests done. I had an MRI on my head and my neck and everything came back negative,” Day said.
“So I have no idea what that was, other than I just may have been exhausted. I was training so hard, I was doing two-a-days every day coming into tournaments and then on top of it I was doing practice, playing competitive golf and then trying to balance that with family as well.
“It’s just a full-time kind of gig there and I think I just ran out of gas and I wasn’t feeling good, so I had the shakes and the tingling up my arms. And the loss of energy and strength was probably caused by that. I’ve got severe sleep deprivation, so I guess that’s part and parcel of having a kid.”
Masters champion Jordan Spieth, who was playing with Day and carded a 67 to set the clubhouse target on five under, told reporters: “I was walking with him, just turned around and he was on the ground. I think he had a dizzy moment and slipped. He could barely even walk when he stood up.
“He did not say much about it, we just went about our business and cleared the scene and the cameras to make sure he could get what he needed.”