SPRINGFIELD-Jimmy Walker birdied five of his first 10 holes to seize a two-stroke lead early in Thursday's first round of the PGA Championship while Rory McIlroy and Phil Mickelson struggled.

Walker, a 37-year-old American who missed the cut in three of his past four major starts, was on five-under through 11 holes at par-70 Baltusrol after starting his round off the 10th tee. Brian Stuard, a 143rd-ranked American who won his first US PGA Tour title this year at New Orleans, was second on four-under after 14 holes.

Sharing third on three-under were England's Paul Casey after 12 holes, Argentina's Emiliano Grillo through 14 holes and American Jamie Lovemark after finishing the back nine. Casey had three back-nine birdies to start, took a double bogey at the first then followed with back-to-back birdies. Mickelson, a 46-year-old American left-hander who was second two weeks ago at the British Open, and McIlroy, who shared fifth at Royal Troon, each made three bogeys on the back nine -- their first nine holes of the day -- to quickly fall eight strokes adrift.

World number one and defending champion Jason Day of Australia, playing alongside Mickelson and McIlroy, held together at one-under through his first nine holes thanks to a 15-foot birdie putt at the 11th. The star trio was the first morning group to start on the back nine that failed to produce at least one sub-par score on the reachable par-5 18th hole. Mickelson had trouble finding fairways and greens while McIlroy's short game was hurting his hopes. Day was battling illness and fatigue. The 28-year-old Australian, a seven-time winner in his past 18 starts, took extra rest this week and played only one practice round.

Sunny skies greeted the early starters from the field of 156, the strongest lineup of talent in any event based on world rankings since their 1986 creation. The 7,428-yard, par-70 layout features only two par-5 holes, those at 17 and 18 to ensure drama and eagle chances to the end in the quest for the top prize of $1.8 million (1.6 million euros).

McIlroy won the Irish Open and has nine other top-10 showings in US and European Tour events this year, including a share of fifth at the British Open two weeks ago. "I feel like my game is in good shape," McIlroy said. "I can hit driver off the tee and from there, if I drive it well, I feel like I have a big advantage." Mickelson won the 2005 PGA at Baltusrol and comes in off his 11th runner-up showing in a major, having lost a duel with Sweden's Henrik Stenson at the British Open.

At 46, the US left-hander would be the fourth-oldest major winner if he claims the title. Stenson, US Open winner Dustin Johnson of the United States and Masters winner Danny Willett -- this year's major winners -- tee off in the afternoon feature group off the first tee, with only three final trios behind them. Just in front of them are England's Justin Rose, American Patrick Reed and South African Louis Oosthuizen.

And the trio teeing off ahead of that offers two-time Masters champion Bubba Watson, another American in two-time major winner Jordan Spieth and Spain's Sergio Garcia, trying to win his first major title in his 73rd try and 70th consecutive major start dating to the 1999 British Open. "If it doesn't happen, it's not going to change my life," Garcia said. "I'm not going to go in a cave and stay there until I die because I didn't win a major or anything like that. It's not that serious. "I'm not going to lie. It would be nice to get at least one. But it's not the end of the world."