Jon Rahm's US Open preparations have been thrown into disarray after he was forced to withdraw from the Memorial Tournament after testing positive for Covid-19.

The defending champion had fired a sensational third round of 64 on Saturday to establish a six-shot lead before being informed beside the 18th green at Muirfield Village that he had returned a positive test.

Rahm had been undergoing daily tests after being told on Monday that he had come into close contact with someone who had tested positive.

But while each of his previous tests had come back negative, his latest one was positive and he must now isolate until June 15, two days before the US Open gets under way at Torrey Pines, where he won his first PGA Tour title in 2017.

The world number three later wrote on Twitter: "I'm very disappointed in having to withdraw from the Memorial Tournament. This is one of those things that happens in life, one of those moments where how we respond to a setback defines us as people.

"I'm very thankful that my family and I are all OK. I will take all of the necessary precautions to be safe and healthy and I look forward to returning to the golf course as soon as possible.

"Thank you to all of the fans for their support and I'm looking forward to watching the showdown tomorrow afternoon with you all."

Before this weekend, the last time a tournament leader on the PGA Tour withdrew before the start of the following round was when Hunter Mahan pulled out of the 2013 RBC Canadian Open because his wife had gone into labour.

On the European Tour, Padraig Harrington held a five-shot lead after 54 holes of the Benson and Hedges International Open in 2000, but was disqualified before the final round after it emerged he had not signed his scorecard on day one.

Tournament host Jack Nicklaus expressed sympathy for Rahm, but insisted there was no option but for the Spaniard to withdraw.

"There was no choice for the TOUR or Jon," Nicklaus wrote on Twitter. "On behalf of the Memorial Tournament, our hearts go out to Jon and his family, as well as all the patrons who witnessed a spectacular round by Jon—only to be negated by this horrible pandemic our world continues to endure."

A statement from the PGA Tour read: "On the evening Monday, May 31, the PGA Tour notified Jon Rahm that he was subject to contact-tracing protocols, as he had come in close contact with a person who was Covid positive.

"Per the Tour's Covid Health & Safety Plan, Rahm was given the option to remain in the competition and enter our tracing protocol, which includes daily testing and restricted access to indoor facilities. Rahm has remained asymptomatic.

"Rahm has tested negative every day, but his most recent test – which was performed after the conclusion of his second round (rain delayed) and before the start of his third round – returned positive at approximately 4.20pm (local time) while Rahm was on the golf course.

"The PGA TOUR's medical adviser requested a confirmatory test on the original sample, which came back at 6.05pm, and was also positive.

"The PGA Tour's medical adviser notified Rahm immediately upon completion of his round, and under Tour protocols, he will be withdrawn from the competition.

"Rahm is now in isolation and, in accordance with CDC guidelines, he will need to remain in isolation through Tuesday, June 15."

Rahm's playing partners in round three, Patrick Cantlay and Scottie Scheffler, have both previously contracted Covid-19.

"I (have) got to imagine I have antibodies, so I don't feel too concerned," said Cantlay, who shares the lead with Collin Morikawa on 12 under par following Rahm's withdrawal.

Scheffler added: "I don't feel very concerned. I had Covid. I'm not going to say whether or not I got vaccinated."