Patrick Reed set the clubhouse target on day one of The 145th Open Championship as South African Haydn Porteous made a big move through the field.
Reed's opening 66 equalled the record for the lowest opening round in an Open Championship at Royal Troon on a day of perfect scoring conditions on the Ayrshire coast.
The American's five under par total gave him a one-shot lead over countryman Justin Thomas but Porteous turned in 30 to get to six under with the help of two eagles on the front nine.
The 22 year old won his maiden European Tour title at the Joburg Open in January but with the back nine providing a much sterner test than the front in Scotland, he still had some work to do to get into the clubhouse at the top of the leaderboard.
Reed is without a European Tour win since the 2014 WGC - Cadillac Championship but lost a play-off to Kristoffer Broberg at the BMW Masters last year and gave a strong showing at last week's Aberdeen Asset Management Scottish Open.
The opening holes were providing the best of the scoring and Reed was one of a host of players to take advantage, holing his approach to the par four third before making further gains on the fourth, sixth and seventh.
A bogey on the tenth halted his momentum but he picked the shot back up on the 12th. Another bogey followed on the 13th but a brilliant approach to the last set up a closing gain.
Porteous took advantage of the two par fives on the front nine as he eagled the fourth and sixth and he then followed that with birdies on the seventh and eighth.
His putt on the ninth looked like it was going to see him go out in 29 but a late turn saw him settle for 30.
Thomas began his round with four straight birdies and another gain on the seventh saw him reach the turn in 31. A double-bogey on the 15th slipped him back but the 23 year old got back to four under with a closing birdie.
Soomin Lee and Richard Sterne were also at four under playing the back nine with Justin Rose a further shot back after a 68.
The Englishman turned in 35 and then picked up birdies on the 15th and 16th to move up the leaderboard.
Jamie Donaldson was then in the clubhouse at two under, leading a large group of players still out on the course including World Number Four Rory McIlroy.
Louis Oosthuizen signed for a level par 71, the highlight of which was a hole-in-one on the short 14th.
