Jordan Spieth held a clutch par putt on the 18th hole to retain the lead at the halfway stage of the Masters, but Rory McIlroy was only one shot back and set to form part of a mouth-watering final pairing on Saturday after the reigning champion slipped back to the field in fiendish conditions.
Spieth led by five at the halfway stage 12 months ago on his way to a stunning wire-to-wire win and threatened to turn this year's tournament into another procession when he opened up the same advantage through three holes on Friday.
However, as a combination of swirling winds, tough pin positions and devilishly fast greens made scoring increasingly difficult, the world number two was forced to work extremely hard just to complete a 74 - his first over-par round at Augusta.
Bogeys at the 16th and 17th left Spieth, who had earlier four-putted the fifth, one ahead of playing partner Bryson DeChambeau and McIlroy with one to play.
Amateur DeChambeau (72) undid much of his excellent work with a painful seven at the last, while Spieth got up and down from a greenside bunker to avoid a third dropped shot in as many holes and remain above McIlroy at four under. The Texan has now held top spot for six successive rounds in the Masters, but has 21 players within five strokes of his score heading into round three.
Scott Piercy (72) and Danny Lee (74) are just two off the lead, with Brandt Snedeker (72), Hideki Matsuyama (72) and Soren Kjeldsen (74) a shot further back.
Spieth's troubles on the back nine, meanwhile, ensure the likes of Dustin Johnson (71) and world number one Jason Day (73) remain very much in contention at level par and one over respectively.
McIlroy, seeking to complete a career Grand Slam with a fifth major title, was one of only four players to break par in round two after overcoming troubles of his own in a 71.
After making up early ground with birdies at the second and third, the Northern Irishman dropped three shots over the next two holes.
However, after bogeying 11, McIlroy finished with a flourish, defying the challenging conditions to birdie both par fives on the back nine before holing a superb long-range putt for a two at the short 16th.
Spieth initially rallied after his six at the fifth, but struggled to keep his emotions in check when back-to-back bogeys at nine and 10 were followed by his group being placed on the clock.
A fine birdie at 15 looked to have settled the leader, only for two more dropped shots to follow, leaving the tournament wide open despite Spieth's gutsy par save on 18.
DeChambeau outshone his professional colleagues for the majority of the second round with a stunning display and was four under through 15 as a result of his sixth birdie of the day.
Yet there was misery for the youngster at the 18th as he lost his drive left and then pulled a second tee shot before eventually holing out for a triple bogey that dropped him back to even par.
At the other end of the field, 2012 and 2014 champion Bubba Watson (75) sneaked into the weekend at six over, but three-time winner Phil Mickelson came back in 40 to card 79 and miss the cut by a stroke.