LA QUINTA, California - Long-hitting American Jason Kokrak recorded two eagles in ideal scoring conditions as he charged into a three-way tie for the lead in Thursday's opening round at the Humana Challenge. While twice former champion Phil Mickelson struggled with his putting en route to a level-par 72, Kokrak fired a flawless 63 at La Quinta Country Club, one of three venues hosting the pro-am celebrity event. The beefy Kokrak took advantage of a picture-perfect day in the California desert, covering his last nine holes in four-under 32 to finish level with compatriots Roberto Castro and James Hahn. In-form PGA Tour rookie Russell Henley, who won last week's Sony Open in Hawaii, was a further stroke back after a 64, along with fellow American Doug LaBelle II and Australians Aaron Baddeley and Greg Chalmers. On a sublime, sun-splashed day in the California desert, Kokrak made a sizzling start with a birdie on his opening hole, the par-four 10th, followed by an eagle at the 11th. "I just got off to a good solid start," he told reporters after mixing five birdies with his two eagles. I just tried to play solid from tee to green, put it in the right spots, put it in the fairways, and out there that's key. Today couldn't have been a better day. Kokrak said he was feeling much more comfortable with playing conditions on the PGA Tour after completing his maiden season on the U.S. circuit in 2012. "As a rookie out here last year I didn't know any of the golf courses, so coming out here now it's a little bit more relaxed," he added. It's easier for me to get into a little bit of a routine. I don't have to go out there and learn every little detail. I can come out here and just concentrate on my game. Castro, who started out on the Nicklaus Private course, birdied his first two holes before surging into a share of the lead with five birdies on his homeward nine. "I got off to a good start, birdied the first two with kick-ins basically, and then just played well after that," the 27-year-old said. I didn't make any long putts until the 18th hole where I rolled in like a 40, 50-footer. This tournament, you kind of got to get it going and keep it going. If you don't make a bunch of birdies you're kind of left behind. While most of the players in the field made the most of the low-scoring conditions, Mickelson struggled on the greens as he battled flu-like symptoms for a 11th consecutive day. "My expectations were to be really low today but I putted terrible," Mickelson, tournament winner in 2002 and 2004, said after totalling 30 putts on the La Quinta layout. I had a bunch of six-footers I missed, my rhythm was off. Also making a poor start to the third event on the 2013 PGA Tour was fellow American Mark Wilson, the defending champion, who laboured to a five-over 77 at La Quinta.