Robert Streb produced a stunning 63 to join Jimmy Walker at the top of the US PGA Championship leaderboard, as reigning champion Jason Day moved ominously into contention with a remarkable second-round charge at Baltusrol.
After double-bogeying the seventh hole on Friday to slip back to level par for the tournament, world number one Day put together a remarkable run of seven birdies in the next eight holes, on a course softened by heavy morning rain.
Three closing pars saw the Australian complete a 65 to share third with Argentina's Emiliano Grillo (67) at seven under.
Walker and Streb are two shots better off, the latter claiming a surprise share of the lead by shooting 63 and becoming the latest in a long line of players to equal the lowest round in major history.
The unheralded Streb recorded eight birdies, including three in his last four holes, to match Walker, who followed his opening 65 with an impressive 66 despite a nervy finish culminating in a bogey on the last.
Open champion Henrik Stenson is just three off the pace and very much in contention for a second successive major win. The undoubted highlight of a second successive 67 for the Swede was a stunning approach to the par-five 18th, his ninth hole, with his trusty three-wood that led to an eagle.
That shot proved the catalyst for a Stenson surge, with birdies following on the first and third, but his efforts paled in comparison to Day's subsequent birdie blitz.
I am just laughing because this is silly. @JDayGolf-- Ellie Day (@ellielaneday) July 29, 2016
After picking up shots at the eighth, ninth, 10th, 12th and 13th, Day rolled in a mammoth putt on the 14th before a laser-like approach to 15 continued his glorious run.
At that point, it looked as though Day may even catch Walker, who had briefly pulled clear of Grillo at the summit with three birdies in a row from the 12th.
Day could not make up further ground, though, and Streb unexpectedly earned a place in Saturday's final group with his strong finish.
Patrick Reed (65), Brooks Koepka (67) and 2010 champion Martin Kaymer (69) are four shots off the leading pair, while Hideki Matsuyama (67) and Rickie Fowler (68) are among those at four under and Jordan Spieth (67) sits six back.
Phil Mickelson made the cut at one over despite beginning his second-round with a shocking seven, but two-time US PGA champion Rory McIlroy joined the likes of Sergio Garcia, Matt Kuchar and U.S. Open winner Dustin Johnson in bowing out.
A birdie at 17 boosted McIlroy's bid to sneak in for the weekend, but he then bogeyed the last and a 69 left him at three over.