Phil Mickelson told his critics to "toughen up" after admitting he deliberately hit a moving ball to gain an advantage in the third round of the US Open.

Mickelson, who was already four over par for the day on his 48th birthday, badly overhit a putt on the 13th green which looked in danger of rolling off the putting surface.

The five-time major winner prevented that from happening by running after the ball and hitting it while it was still rolling, a breach of rule 14-5 which incurs a two-shot penalty.

Former US PGA champion Steve Elkington called for Mickelson to be disqualified, writing on Twitter: "Absolutely 100% conduct unbecoming ... hes trying to embarrass the @USGA DQ his ass."

And LPGA player Christina Kim wrote on Twitter: "I have never done anything so ghastly. I'm curious to know what sort of "logic" caused him to do that. I'm in shock."

However, Mickelson, who eventually made a 10 on the 13th in a round of 81 which equalled his highest score in the US Open, said: "If someone is offended I apologise, but toughen up.

"Knowing the rules is never a bad thing. You always want to use them in your favour. I know the rules and the ball was going to go off in a bad spot. I did not feel like continuing going back and forth. I would still be out there potentially.

"I've wanted to do it many times before and finally did. I should have done it a couple of times on 15 at Augusta. That would have saved me a shot or two back then."

Mickelson and playing partner Andrew Johnston were laughing about the incident as they walked off the green and Mickelson added: "How can you not laugh? It's funny. I just wanted to get to the next hole and did not see that happening without the two shots."

Commentating on the incident for Fox, former Ryder Cup captain Paul Azinger said: "That's the most out of character I have ever seen Phil. I think he just snapped. I'm sure he is going to regret that."

And two-time US Open champion Curtis Strange added: "He will feel some embarrassment because there are a lot of people and kids watching that really admire this guy."

The incident overshadowed the early stages of the third round by the leading players, with Dustin Johnson seeing his four-shot lead cut to a single stroke.

Johnson, who was an odds-on favourite to become only the second player after Tiger Woods to win while ranked world number one, suffered his first double bogey of the week on the second and dropped another shot on the fourth.

That allowed Justin Rose and Henrik Stenson to close the gap thanks to birdies on the third and fifth respectively.