Spieth, who was tied for the lead in the Open Championship at the end of Thursday and started off strong on Friday, holed this beauty on No. 10 to save par.
This is the fourth time the 23-year-old Texan has at least a share of the 36-hole lead in a major.
Within minutes of Jordan Spieth's birdie putt at the 18th hole finding the cup for what would soon be confirmed as a three-shot lead over Matt Kuchar, a question started to buzz around the media centre: "What do you think, is it over?" But he missed a short par putt on the 16th.
He told BBC Sport: "I am proud of myself for hanging in there, staying as positive as I could".
"It's completely different than any tournament we play in the United States, and just kind of soaked it in for a second, and said, 'This is pretty cool'".
Both took time to stop and soak in the best walk in golf, a full grandstand surrounding both sides of the 18th. I've done the same thing for years, " Koekpa said. It's amusing, I'll play with my dad and shoot 75 every time or higher.
"I'm ready to go out there toe to toe with anyone this weekend".
"But it's fun, it's an Open, it's what it's supposed to be".
The good news is the weather - rainy and breezy in the morning - is expected to get better as the day goes on.
The past seven majors have been won by first-time winners, although Royal Birkdale has a quirky habit of rewarding former champions and consecutive winners. Miller was the first to shoot 63 in a major at the 1973 U.S. Open at Oakmont.
A week shy of his 24th birthday, a player so often hailed for his maturity has carried the air of a grizzled veteran in Southport, totally prepared for whatever challenges he might face.
"It's a mind game that you play with yourself there", said Spieth, who is looking to add to his two breakthrough majors from 2015.
"I believe that we can do this".
Kuchar's one-over-par round of 71, in the gusty winds, was only slightly soured by a bogey on the 18th.
Constant 25-mph winds and gusts of up to 40 miles per hour took its toll on the best golfers in the world on Friday as scores ballooned.
Ian Poulter was three under par after 16 holes, with world number two Hideki Matsuyama also -3 after eight holes. "Today is my day". I get to kick back in the afternoon and watch the guys just try to survive [which] is pretty much all you can do.
"Conditions were really hard today", he said. Almost opposite wind of what we had yesterday. There are 18 holes still to play and, even at the course which yielded the first 62 in men's major history, disaster is lurking.
"And this wind, it felt like every hole was a crosswind hole".
He is not alone, though, with reigning US Open champion Brooks Koepka also shooting 65. "So it hasn't really sunk in, I don't think yet".
But he said he feels like he is still improving and hasn't thought about quitting the game. "I'm excited. I'm pumped up". "I feel like I am in a good place right now so I'm happy and I'm excited".
On a good morning for Europeans, England's Matthew Fitzpatrick, Belgium's Thomas Pieters and defending champion Henrik Stenson of Sweden all fired one-under-par rounds of 69.
The American felt a bogey at the 18th, after finding one of Birkdale's 123 bunkers, summed up his hard day. I just wanted to get back inside the ropes. Even though those players in the afternoon wave got a little wet, they likely got the better of the round's conditions.
"I wish I could blame it on the wind". And I remember being on the course thinking - at one point on the 12th hole Saturday, I had 129 yards to the pin and hit a 6-iron in.
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