World number three Jon Rahm's Saturday began with a hole-in-one which helped him take a six-shot lead at the USPGA Memorial Tournament in Ohio.
However, it ended with the Spaniard, 26, withdrawing from the tournament after being told he had tested positive for Covid-19 as he walked off the 18th green.
Defending champion Rahm looked set to claim the $1.67m (£1.18m) first prize.
He said he was "very disappointed" to have to withdraw.
Rahm reflected: "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."
The PGA Tour said Rahm was given the news as he finished his round as part of "Tour protocols".
A Tour statement 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 Eastern Time while Rahm was on the golf course. The PGA Tour's medical advisor requested a confirmatory test on the original sample, which came back at 6.05pm ET, and was also positive."
The Tour said Rahm now has to isolate for 10 days.
"While this is an incredibly unfortunate situation, throughout 50 events since the PGA Tour's return to golf, there have been only four positive tests (including Rahm) within competition; Rahm is the first positive, asymptomatic case as part of the Tour's routine, contact-tracing protocols," the statement added.
Rahm's hole-in-one came on the par-three 183-yard 16th hole as he completed a storm-delayed second round on Saturday morning.
That gave the 26-year-old defending champion a two-shot lead after two rounds over American Patrick Cantlay.
Rahm extended that to six shots after a round of 64 put him 18 under for the tournament, with Collin Morikawa and Cantlay joint second.
After Rahm's misfortune, the pair now hold a three-shot lead over the chasing pack.
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