Steve Stricker fired a 10-under par 63  to take a five-shot lead at the halfway stage of the US PGA Tour's season-opening Tournament of Champions. Stricker, at number six in the world the highest-ranked player in the 27-man field, had eight birdies and an eagle without a bogey as the American took his 36-hole total to 15-under par 131. Webb Simpson posted his second straight 68 to sit second on 136, while South Korean-born American Kevin Na climbed into third place on 137 with a sparkling 64. Stricker was already six-under for the day when he eagled the par-five 15th, where he hit a three-wood to 12 feet. He added two more birdies at 17 and 18. "This game is never easy, but I had it going today," Stricker said. "I felt like I was going to make every putt I looked at there for a while and I gave myself a lot of opportunities -- which is the key." Na thrust himself into contention with an eagle-eagle finish. He started the day even and was still at even par for the tournament through eight holes. Na played his last 10 holes in nine-under par, including his back-to-back eagles. At 17, Na holed out with a five-iron from 221 yards and at 18 he drained a 10-footer. Na's back-nine 29 equalled the back-nine record for the par-73 Plantation Course set by Vijay Singh in 2004. "At 18 I hit a beautiful second shot into the green and I just kept telling myself 'for 29, for 29.' As soon as I hit it, I knew it was in," Na said. "That was exciting. That was a lot of fun. All of a sudden on the back nine I started getting it together." The field, open only to last season's title winners, was reduced to 27 on Friday when former US Open champion Lucas Glover withdrew with a sprained knee. Three of the four reigning major champions had already opted out with Rory McIlroy and Darren Clarke of Northern Ireland and Charl Schwartzel of South Africa bypassing the $5.6 million event. Scotland's Martin Laird and first-round leader Jonathan Byrd shared fourth place on 138. Laird rebounded from a double-bogey at 17 with a birdie at 18 for a three-under 70, while Byrd posted a two-under 71. Despite his comfortable cushion, Stricker said he wouldn't get ahead of himself with two rounds remaining in the event that finishes on Monday. "We're halfway through," he said. "We've got a long ways to go yet."