One of eight players unexpectedly invited to the season's second major despite plying his trade on the Saudi-backed rebel tour, Herbert stormed up the leaderboard with a second-round four-under-par 67 at Valhalla.
He sits six shots behind halfway leader Xander Schauffele, who backed up his record-breaking 62 with a 68 to retain a one-stroke buffer over fellow American Collin Morikawa (65).Â
"I felt like I got off to a solid start on the front nine and a good finish there with birdies in the last three holes in the front nine," Herbert said.
"That put me in some good momentum for the back nine where you weren't really scrambling to make cut anymore.
"You're more able to work your way up the leaderboard going into the weekend.
"That's given me the opportunity to be in the position I'm in now so hopefully we can build into contention on the weekend."
Three brilliant chip-ins helped Lee to a five-under 66 that also sent the Perth prodigy rocketing up the leaderboard.
He joined Day, who had a five-birdie 67, at four under for the championship and with a fighting chance of a weekend charge to the Wanamaker Trophy.
Cam Smith stalled somewhat but still joined Lee and Day at four under after closing his round of 70 with an all-important birdie at the last.
After starting the day over par, in 85th spot and in danger of missing the halfway cut, Lee birdied his first and third holes before igniting his round with his first hole-out from off the green on the par-4 16th.
He also chipped in on the second and seventh holes after starting on the back nine.
"It was just one of those days where it kind of went my way. Yeah, they just dropped," Lee said.
"That definitely helped the scorecard. But a bit more solid than yesterday. Yesterday I just had a few missed iron shots, and today it was trending in the right direction.
"Yeah, the chip-ins were nice, obviously, and those were three birdies, so yeah, it was most of the score - 16 was pretty special.
"It was in deep rough into the grain and I was just trying to get it on the green close, and it ended up going in."
Champion in 2015 and runner-up in 2016, Day also gave himself an outside chance of contending on Sunday with a solid round containing just one bogey.
But Adam Scott (72-73) missed the cut at three over, as did Cam Davis (78-71) at seven over.
The rallies from Herbert, Lee and Day came hours after world No.1 Scottie Scheffler was arrested and released by Louisville police following an early-morning traffic incident en route to the course.
Despite being booked on four charges, including second-degree assault of a police officer after his vehicle allegedly dragged an officer to the ground and injured him, Scheffler held his nerve to carve out a 66 to be tied for fourth at nine under.
The Masters champion is three strokes behind Shauffele as he continues his quest for the second leg of golf's grand slam.
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