The Masters took an intriguing twist as reigning champion Jordan Spieth saw his lead cut from five strokes to two during the second round, with four-time major winner Rory McIlroy and amateur Bryson DeChambeau the men applying the greatest pressure.
Seeking to follow up his stunning achievement in claiming a wire-to-wire win at Augusta 12 months ago, Spieth birdied two of his first three holes on Friday to reach eight under and hold a commanding advantage at the top of the leaderboard.
However, with increasingly strong swirling winds and lightning-fast greens ensuring Augusta offered a stern challenge, the 22-year-old Texan ran up a four-putt double bogey at the fifth.
Spieth looked to be back on track when an excellent par save from 17 feet on the next hole was followed by a birdie at the long eighth, but more trouble lay ahead for the long-time leader as he carded back-to-back bogeys at the ninth and 10th.
In addition, Spieth was clearly unsettled by his group being placed on the clock as they came up the 11th, where playing partner DeChambeau rolled in his fourth birdie of the day to get within three.
When DeChambeau picked up another shot at 12, courtesy of a sublime tee shot, he was alongside McIlroy at three under and only two off the lead. However, Spieth could take heart from clutch par putts on 11 and 12.
McIlroy surged into contention with a back-nine charge, birdies at 13 and 15 followed by another at 16, where the Northern Irishman held a left-to-right putt from 39 feet.
A superb up-and-down from 87 yards rescued a par for McIlroy on the last, ensuring he completed a 71 to claim the clubhouse lead.
Scott Piercy (72) and Danny Lee (74) were also finished for the day and likely to be right in the mix heading into the weekend with Augusta showing its teeth.
Lee's two-over round was admirable given he endured a host of mishaps. The New Zealander double-bogeyed the third and found water at the 12th and 13th before three-putting each of the last two holes.
World number one Jason Day had to settle for a 73, but was by no means out of contention at one over as Spieth came back to the field.