A shock blunder from Jordan Spieth gave the chasing pack hope after the defending champion had threatened to turn the Masters into a procession for the second year in a row.
Spieth, who was second on his debut in 2014 and claimed a first green jacket with a record-breaking wire-to-wire victory 12 months ago, held a two-shot lead after a flawless opening 66.
And the world number two swiftly moved five shots clear of the field with two birdies in the first three holes of his second round as he looked to become just the fourth player after Jack Nicklaus, Nick Faldo and Tiger Woods to successfully defend the year's first major championship.
Spieth, who set records for the highest number of birdies (28) and lowest 36 and 54-hole totals in Masters history last year, holed from 14 feet on the first and just two feet at the third, after surprisingly missing from seven feet on the second.
However, the 22-year-old then four-putted the fifth from 50 feet run up a double-bogey, bringing back unhappy memories of doing the same on the eighth hole in the final round of the Open at St Andrews last year. Chasing the third leg of an unprecedented grand slam, Spieth eventually missed out on the play-off by a single shot.
Wow, Spieth four-putts the fifth, missing from seven feet, four feet and three feet.*— Steve Elling (@EllingYelling) April 8, 2016
*According to Masters site. pic.twitter.com/jOrz0PtSyn
Unfortunately for Rory McIlroy, his own problems meant he was still six shots off the pace after Spieth's unexpected mistake.
McIlroy had been within two of the lead on Thursday before bogeys on the 16th and 18th and it was a case of deja vu for the world number three on Friday, who needs to win a green jacket to complete the career grand slam.
Birdies on the second and third took McIlroy to four under par, only for the 26-year-old to double bogey the fourth, where he found sand off the tee and compounded the error by three-putting from 20 feet for a double bogey.
A poor chip on the next led to another dropped shot before the Northern Irishman crucially saved par from 12 feet on the sixth, having badly misjudged his birdie attempt.
However, a run of pars was followed by another dropped shot on the 11th which left the four-time major winner back on level par.
Sergio Garcia had also been four under after birdies on the second, third and eighth, only for the Spaniard to take six on the ninth and drop back to two under alongside Soren Kjeldsen, Shane Lowry, Paul Casey and Justin Rose.
Playing alongside Spieth, Casey also got to four under with a birdie on the second, but three-putted the third after a misjudged approach in the swirling wind and also dropped a shot on the next.