Andrew Johnston described Phil Mickelson running to putt a moving ball at the U.S. Open as "a moment of madness" and "something you might see at your home course with your mates".

A flustered Mickelson sparked controversy when he chased after his putt at the par-four 13th and struck the ball back towards the hole as it rolled down the slope at Shinnecock Hills.

The five-time major champion was given a two-shot penalty for the rule breach on his 48th birthday, resulting in a 10 and a horror 11-over third round of 81 to fall back to 17 over.

Johnston, who went 12 over, said he and Mickelson could not help but laugh over the contentious incident as they struggled in such demanding conditions.

"[It was] just one of the mad moments, really," said the charismatic Englishman.

"We were both obviously really struggling. It was funny because I didn't think he hit that bad of a chip shot from behind the green in the first place. And as it released and went through, I thought wow, that is seriously quick.

"So to knock that past and then hit the putt going back towards back off the green, I think it's just one of the moments where you're not thinking about it. It just happens and he just did it.

"It wasn't a thing of saying, 'I've had enough or this and that.' It just happened."

Johnston added: "I said [to Mickelson], 'That's one of the strangest things I've ever seen,' and then just started laughing, to be honest. I said, 'I'm sorry, but I've got to laugh at this.'

"I think we were both happy to get off the golf course."

Mickelson said he did not mean to be disrespectful and Johnston had no issue with the birthday boy.

"I've never seen anything like it. It's something you might see at your home course with your mates or something. I think it's just a moment of madness," he added.

"But it's nothing disrespectful to me or to the U.S. Open or anything. It's just one of them things that just happened."