* Yang fires 64 for a two-way tie at the top
* Watson catches Yang late in the day
* Defending champion Mickelson cards a 71 (Updates at end of round)
Jan 30 (Reuters) - South Korean Yang Yong-eun birdied six of his last nine holes to surge into a share of the lead in the Phoenix Open first round while Phil Mickelson launched his title defence 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.
Yang, who has not recorded a single top-10 on the U.S. circuit since he tied for sixth at The Barclays in 2011, was delighted with his up-tick in form.
"I have been playing like 'you-know-what' for a long time now," the Korean told PGATour.com through an interpreter. "I have been adjusting my swing for a long time ... and I think it's finally paying off.
"I missed some good chances on the front nine so I was a little bit angry with myself but I've been playing some solid golf for the last four-and-a-half rounds so I think it's good."
Yang's lead held up for most of the day until he was joined at the top by 2012 Masters champion Bubba Watson, who birdied four of his last six holes for a matching 64.
Americans Pat Perez, Kevin Stadler, William McGirt, Harris English and Chris Kirk, along with Australians Greg Chalmers and Matt Jones, were a further stroke back after opening 65s.
SETTING THE PACE
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 the 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 overdo it and practise 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."
American Hunter Mahan, the 2010 champion, opened with a five-birdie 66 while England's former world number one Lee Westwood carded a 67. (Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue)