Walker added a 66 to his opening 65 to post a nine-under-par score of 131 and equal the lowest halfway total in the year’s final major - but bogeyed the last to cost himself a share of the all-time major record of 130 held by Martin Kaymer, Brandt Snedeker and Nick Faldo.
The 37-year-old was then joined at the top of the leaderboard by compatriot Streb, who carded eight birdies and a solitary bogey to become the 28th player to shoot 63 in a major.
Defending champion Jason Day and Argentina’s Emiliano Grillo were two shots behind, Day recovering from a double bogey on the seventh to birdie seven of the next eight holes in a 65.
And Open champion Henrik Stenson was just a shot further back in his bid to win two major titles in the space of three weeks after carding a second consecutive 67.
However, two-time winner Rory McIlroy missed his second halfway cut in three majors and his first in the US PGA after a bogey on the 18th, the Northern Irishman taking three attempts to find the green from thick rough between two bunkers.
Walker was unaware of the record possibilities as he stood on the 18th tee and promptly drove into the water to the left of the fairway and compounded the error by three-putting from long range.
“The finish wasn’t what I was looking for, but it’s fine,” the five-time PGA Tour winner said. “I think you take 66 at any tournament all day every day.
“So it’s good. It’s really cool. You’re at the PGA Championship. You are at the top. It’s what you are here to do. It’s fun.
“I’ve won a few times. I’ve won different ways. I feel like I’ve done a lot in golf. So this is one more little thing here that I haven’t really experienced before - but I’m pretty whooped.
“I’m going to go home and play with the kids and get them to bed. I’m pretty sure I’m going to fall right asleep and be just fine.”
Streb has not recorded a top-10 finish since last year’s US PGA at Whistling Straits, but compiled the third 63 in the space of 16 days after Phil Mickelson and Stenson achieved the feat in the Open at Royal Troon.
“I’m happy to join the club that seems to be ever-growing,” the 29-year-old said. “Most of the year has been a big struggle...since the PGA last year.
“It’s felt like I found something with my golf swing the last two weeks. I started hitting it kind of solid, feeling a little bit like my golf swing again, and obviously made a bunch of putts today.
“My putt on eight (for birdie) was a fairly pathetic effort, but I made up for it on nine.”
Day came into the week “under the weather” and only had one practice round after spending Tuesday evening in hospital after his wife Ellie suffered an allergic reaction.
But the world number one revealed an X-rated pep talk he gave himself after the double bogey on the seventh - where he needed four to get down from a greenside bunker - had done the trick.
“Just a few swear words inside my head, like ‘What are you doing?’” the 28-year-old said.
“I played that hole so bad that it was really frustrating for me. Why are you doing that? Why are you giving shots to the field when you don’t need to do that?
“It’s always good to be in contention at a major championship. It would be nice to get my second major and get that under the belt. Right now I’ve got to focus on resting up and be as patient as possible over the weekend.”
Stenson only had a week to drink ‘’champagne, champagne and champagne’’ out of the Claret Jug due to a condensed schedule to accommodate golf’s return to the Olympics.
But he insisted he felt ‘’pretty fresh’’ at the start of the week and believes he will feel the benefit of having ended his major drought if he gets into contention on Sunday.
‘’Maybe I’ll run out of steam at some point, but up until then, I’ll just keep trying my best,’’ he said.
‘’I expect to be tired at some point, whether it be after this week or after the Olympics. I know I’m going to sleep for a week after the Ryder Cup, there’s no two ways about that.
‘’I had a second and two thirds at the Open before this year, so I didn’t feel like I had anything to lose there - and I certainly don’t have anything to lose here.
“Confidence-wise, of course it’s not going to be a bad thing, getting that win. Hopefully it makes it easier on Sunday when you know you’ve got one in the bank already.’’
Martin Kaymer already has two majors in the bank after winning the 2012 US PGA and 2014 US Open and a second round of 69 left him on five under par, alongside Patrick Reed and Brooks Koepka.
Another Ryder Cup hero, Welshman Jamie Donaldson, led the British challenge on four under with Paul Casey and Andy Sullivan two under and Andrew Johnston, Tyrrell Hatton, Lee Westwood and Ross Fisher all one under.
Scotland’s Russell Knox was level par after a second straight 70.