South Korean Yang Yong-eun birdied six of his last nine holes to surge into an early one-shot lead in the Phoenix Open first round while Phil Mickelson launched his title defense with a level-par 71 in Scottsdale, Arizona on Thursday.
Yang, who became Asia's first male major winner with his victory at the 2009 PGA Championship where he overhauled Tiger Woods in the final round, mixed eight birdies with a lone bogey to fire a seven-under-par 64 in dazzling desert sunshine.
The 42-year-old Korean, a double winner on the PGA Tour, took advantage of ideal scoring conditions at the TPC Scottsdale and signed off with a superb approach to three feet at the par-four last before he sank the birdie putt.
Pat Perez and Kevin Stadler opened with 65s, while fellow Americans Hunter Mahan, Keegan Bradley, Ryan Moore, Tommy Gainey and long-hitting Jason Kokrak were a further stroke back.
While Yang set the early pace, defending champion Mickelson was happy to complete his opening round with an assorted mix of five birdies, three bogeys and a double after struggling with muscle pain in his lower back over the past two weeks.
"My back's fine, my game was a little rusty," Mickelson, who was diagnosed with and treated for locked facet joints in his back on Monday, told Golf Channel.
"Got off to a poor start, played a couple over (par) and finished poorly with a few over. In the middle of the round, I hit a lot of good shots and had a good little run.
"It just wasn't quite sharp, I wasn't quite focused on every shot the way I need to be and let way too many shots slide today."
Five-time major champion Mickelson double-bogeyed the par-three 12th, his third hole of the day, after hitting his tee shot into water but got his round back on track with birdies at the 13th, 14th, 17th, first and fourth.
The American, who won last year's Phoenix Open with a record-tying 28-under total of 256 after opening with a 60, then faded with bogeys at the fifth, seventh and ninth but preferred to dwell on the all positives in his first round.
"The back feels great, and hopefully I'll come out tomorrow and get a good round," Mickelson said of his game. "I won't overdue it and practice too much.
"I do feel like I've got a good round in me, the game doesn't feel far off even though I made quite a few careless spurts out there."
England's former world number one Lee Westwood and 2012 Masters champion Bubba Watson were among the day's late starters.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue)