The 41-year-old American, who won the prestigious PGA Tour event at Colonial Country Club in 2013, piled up three birdies on each nine to fire a flawless six-under-par 64 on a course softened by heavy rain overnight.

Weekley, a three-times champion on the highly competitive U.S. circuit, signed off by sinking a 13-foot birdie putt on his final hole, the par-four ninth, to finish level with compatriots Jordan Spieth, Kevin Na and Japan's Ryo Ishikawa.

Masters champion Spieth, the world number two and local favourite, sank a 20-foot birdie putt at the par-four 18th late in the day in front of huge galleries to beat the fading light and make it a four-way tie at the top.

American George McNeill and Englishman Ian Poulter opened with 65s while New Zealander Danny Lee, Canada's David Hearn and Americans Ben Martin and Charley Hoffman were among a group of nine players on 66.

Weekley, whose best finish this season was a tie for second at the Sanderson Farms Championship in November, totalled only 27 putts as he surged into early contention after the start of play was delayed by three hours due to the threat of lightning.

"It was good," Weekley, who was in an upbeat mood despite being troubled by a stiff back, told PGA Tour Radio. "I've got good memories here, you know. "I played well today and made a lot of good putts. We read the greens good today, me and my caddie. We had fun."

Na, whose only PGA Tour victory came at the 2011 Justin Timberlake Shriners Hospitals for Children Open, birdied four of his last eight holes to set the early pace before being joined at the top by Weekley, Ishikawa and Spieth.

"It was a good day," said the 31-year-old American who was born in South Korea before moving to the U.S. as a child. "I made a lot of good putts out there. I made a couple of difficult par putts that kept my round going.

"The greens are really good. They are rolling great. Right now, the rough is wet and thick and it's hard to get enough distance out of the rough. The key is to hit the fairway."

Australian Adam Scott, who clinched last year's Crowne Plaza Invitational in a playoff with Jason Dufner, launched his title defence with a 72.

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