And yet Grace, the 29-year-old ranked 35th in the world, later admitted he had been blissfully unaware of what he was about to achieve as he came down the 18th.
In front of thousands of fans, an outstanding putt over the undulations ended up just a couple of feet away, before completing his final par-putt of the day to seal his name in the record books and the impending "amazing Grace headlines". "I promise you", said Grace as his opening gambit.
More than any other player near the top, Spieth knows what Sunday brings.
He then drained more putts for birdies on 14 and 16 before the two-putt on 17 meant he needed just a par from the last - where up to that point there had been just one bogey all day.
Asked how he could not have known, he said: "I honestly didn't".
There have been plenty of 63s shot in major tournaments - a total of 31 recorded by 29 different golfers, most recently by Justin Thomas at last month's U.S. Open at Erin Hills. "And then finishing the round and then finding out what you've done makes it even better, it really does. I really enjoy playing golf with him".
"I think I left four in the heart today that would have got me to 9 (under) if I holed those", Day said after shooting a 5-under-65.More news: Protesters Disrupt MN Mayor's Presser on Woman Shot By Cop
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Going out 10 minutes before them, England's Ian Poulter, runner-up here to Padraig Harrington in 2008, and current US Open champion Brooks Koepka were both three-under overnight. The latter, who turned pro in June, was the story of the Canadian Open previous year, making it to the final group on Sunday as an amateur before settling for a tie for ninth. Three players have shot 62 - Minea Blomqvist in the 2014 Women's British Open, Lorena Ochoa in the 2006 Kraft Nabisco Championship and Mirim Lee in the 2016 Women's British Open.
It wasn't the first time that Grace had gone low in a major.
It is true that this was one of the easiest days in recent Open memory but it was also one of the most exciting with players from way down the leaderboard making their charge. "I was just grinding, grinding, and I just couldn't believe my final round, making so many birdies". "I had no idea that 62 was obviously the lowest ever", he added. My whole thing on the 18th was trying just not to make a bogey. Using his pencil grip, Grace calmly lagged to three feet. The South African didn't know it at the time, but it was the lowest score ever shot in a major championship.
But the Texan added: "Tomorrow will be a day that will be emotionally draining and hard to stay very neutral in the head, but that's probably the most important thing for me to do".
By Sunday evening at Royal Birkdale, Jordan Spieth might have the Claret Jug to bring to Quail Hollow. Hopefully a lot of lives can be changed and can be kind of restored, if I can put it that way.
Sometimes the good guys do win. "We had a good chat and it's been working ever since".