Jason Day famously fell over during the 2015 U.S. Open due to a bout of vertigo, but the world number one was in the vicinity of a more damaging trip on Tuesday in a practice round ahead of this year's tournament at Oakmont.

Day was preparing to play a bunker shot on the sixth hole when the caddie of amateur playing partner Chris Crawford, Bill Henaghan, suffered a tumble that resulted in a broken left ankle.

Medical assistance was swiftly called for, with Day among those to help the stricken Henaghan before long-time Pittsburgh Steelers doctor Jim Bradley, a member of Oakmont who happened to be walking with the group, headed a first-aid assessment.

Describing the unexpected drama, Day told Golf Australia: "He was walking around the bunker pretty quick to get out of my way, I think, and he fell down a little bit and I hear this 'snap' and I thought that sounded weird.

"I turned around and he [Henaghan] said, 'Jason, I've just snapped my ankle'.

"I said, 'What?' and he said it again, 'I've snapped my ankle. I'm OK, so you go ahead'. He was actually asking me to hit.

"I was [saying], 'Mate, I'm not going to hit, I'm going to try to get a medic over here' ... and then he was taken care of.

"Poor fella, I feel so sorry for him. He was probably so jacked to caddie at the U.S. Open, especially at Oakmont, and then to have it done on hole six of a practice round two days before it all starts, that's pretty devastating."

Day's own stumble 12 months ago came as he played his final hole, the ninth, in round two at Chambers Bay. The Australian went on to shoot an impressive 68 in his third round before finishing inside the top 10.