By Mark Lamport-Stokes AUGUSTA, Georgia (Reuters) - World number two Jordan Spieth made an impressive start to his Masters title defense as he birdied three of the first nine holes to take an early one-shot lead in Thursday's opening round. On a relatively calm morning in dazzling sunshine at Augusta National, the 22-year-old American sank a six-footer at the third, a 13-footer at the sixth and a four-footer at the eighth to reach the turn in three-under 33. Casey had completed nine holes, Donaldson and Berger five and Fitzpatrick two. More...