(Amends Els' score in para 14 with official correction) * Spieth goes bogey-free to launch title defence * Says 'one of the best rounds I've scored' * Englishmen Casey and Rose card 69s By Mark Lamport-Stokes AUGUSTA, Georgia, April 7 (Reuters) - Jordan Spieth's love affair with Augusta National continued on Thursday as the reigning Masters champion fired a flawless six-under-par 66 to seize an early three-shot clubhouse lead in the opening round. The American world number two, bidding to become only the fourth player to claim back-to-back Masters victories, took advantage of relatively calm morning conditions to storm to the top of the leaderboard. Shaking off a run of inconsistent form in recent weeks on the PGA Tour, Spieth 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. More...