Australian golfer Jason Day collapsed on the final hole at the end of his second round at the US Open with suspected vertigo.

The 27-year-old was walking to the green when he keeled over, and he spent several minutes on the ground being treated by medical staff.

He opted to finish his round and he sits nicely placed on two-under-par, but there are doubts over his future participation in the tournament.

Day pulled out of the Byron Nelson Championship three weeks ago because of dizziness, and revealed on Tuesday that he has undergone tests to diagnose the problem.

He told reporters: "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.

"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."

Day is currently ranked 10th in the world.