Woods is 2-over par through six holes, Watson finished at 6-under 64. Mickelson and Kenny Perry each fired 66.
Tiger Woods at Firestone Country Club today.Akron -- Tiger Woods has three bogeys and one birdie through the first six holes today, not getting off to the start he wanted in quest of his eighth WGC-Bridgestone Invitational title.
Woods is also after his first win of the season.
Phil Mickelson closed his first round on Firestone Country Club's South Course with a flourish, notching birdies on six of his last 12 holes to finish with a 4-under par 66.
Bubba Watson, like Mickelson a left-handed golfer, fired a 64 and holds a two-stroke lead over Mickelson and Kenny Perry, who carded a 66. Also at 4-under is Bo Van Pelt, who started his day on the back nine has birdied four of his first eight holes.
Masters champion Mickelson has a chance this week to surpass Woods and achieve the world's No. 1 ranking for the first time in his career. Lefty began play on Firestone's back nine, and was 2-over after bogeys on Nos. 14 and 15.
Mickelson birdied the 667-yard, par-5 16th hole. Then, on his back nine, he strung together birdies on hole Nos. 2, 3, 6, 7 and 9.
Watson set up his sterling round with birdies on the 11th through 14th holes.
Chad Campbell and Sean O'Hair shot 3-under 67s. Steve Stricker, Gregory Bourdy of France and Rory McIlroy of Ireland completed their rounds at 2-under, 68. In the clubhouse at 1-under par 69 are Ernie Els of South Africa, Alexander Noren of Sweden and Matt Kuchar.
British Open champion Louis Oosthuizen of South Africa shot a 2-over 72.
Players now at 2-under par include Rickie Fowler (through eight holes), Australia's Adam Scott (eight), Spain's Miguel Angel Jimenez (six) and South Africa's Retief Goosen (four). Ben Curtis of Kent State has carded a 1-under over his first 10 holes and U.S. Open champion Graeme McDowell is 1-under through three.