American Weaver earned his place at the top of the leaderboard with a strong finish claiming three birdies in his final six holes.
One of the four overnight leaders, journeyman professional Jeff Brehaut, dropped a shot but still maintained his challenged as he ended his round on level par.
While Weaver and Brehaut shone some of the more illustrious names struggled.
World No1 Tiger Woods missed an eight-foot par putt at the seventh with his first stroke of the day to fall to two over par while reigning Open and US PGA champion Padraig Harrington also dropped a shot at the hole and then bogeyed the ninth after tangling with the rough off the tee.
Six over became seven at the 12th but the Dubliner sent down back-to-back birdies at 13 and 14 and was five over with four holes to play.
Woods also clawed his way back from two over, with birdies at the 11th and 14th to get to level par but then both major champions double-bogeyed the par-four 15th to swing once more into reverse gear.
With storms forecast for tomorrow and more rain expected on Sunday, play could be being extended into Monday, the day usually set aside for an 18-hole play-off.
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