Defending champion Jordan Spieth fired a brilliant 66 to set the clubhouse target on day one of the Masters Tournament, but a charging Shane Lowry was closing in on the American.
Spieth was unblemished in getting to six under as he hopes to become the first man to successfully defend the title at Augusta National since Tiger Woods in 2002.
He had a daunting chasing pack behind him, however, with Lowry making five birdies in his first eight holes to turn in 31 and sit just a single shot behind, one clear of World Number One Jason Day.
Day had turned in 31 with an eagle and three birdies courtesy of some big hitting and sensational approach play but he dropped a shot on the tenth as Lowry built momentum.
Justin Rose was in the clubhouse at three under alongside Paul Casey, with Danny Lee also three behind the leader after 12 holes.
After a good up-and-down to save par on the first, Spieth birdied the third and sixth before putting his approach on the eighth to four feet for another gain. Further birdies followed on the tenth and 13th, and a closing gain put him in control.
Lowry started with a par but then went on a brilliant run as he picked up shots on the second, third, fourth and fifth before another birdie followed on the eighth.
Day eagled the second after a drive of 326 yards with birdies on the fifth and eighth followed by a tap-in gain on the ninth before he three-putted the tenth.
Rose has never missed the cut in ten visits to Augusta and was full of confidence coming into the week after his runner-up finish last year.
The Englishman made six birdies on his way round but also had three bogeys in a 69, with Casey also dropping three shots with six gains.
Two more Englishmen in Matthew Fitzpatrick and Danny Willett were then in the group at two under with two and nine holes left to play respectively, alongside Scott Piercy and par three competition winner Jimmy Walker.
Lee Westwood signed for a round of 71 while Rory McIlroy, chasing a career Grand Slam of Major Championships, was also one under after eight holes.
