New world No. 1 Vijay Singh stumbled to four bogeys and a triple bogey en route, but battled to card a 1-over 72. He shares second place with Jesper Parnevik (71) and Cliff Kresge (67) at 7-under-par 206.
D.J. Brigman fired a 4-under 67 to move into a tie for fifth at 5-under-par 208. He stands alongside Joe Ogilvie, who posted a 2-under 69.
The field managed to complete rounds two and three on Saturday. Action was delayed due to a five-hour delay in the first round.
Weir was steady throughout. He found the green on each of the first four holes, but two-putted for par each time. The Canadian knocked his second into a greenside bunker at the par-5 fifth at Glen Abbey Golf Club.
He blasted out of the sand, but again two-putted for par. Weir ran his par string to seven before he chipped in for birdie at the par-4 eighth.
Weir's second shot to the par-4 10th came up short of the green and he pitched to 8 feet. However, he missed that putt to slip back to minus-9.
The 2003 Masters champion returned to his steady golf. He ran off seven consecutive pars from the 11th to remain two clear of the field. At the last, Weir carded his second birdie of his round to move to 10 under and a three stroke cushion.
'I had an opportunity to really distance myself, and unfortunately it didn't happen,' said Weir, a seven-time winner on the PGA Tour. 'On the other hand, I didn't putt worth a darn all day and I increased my lead by two.'
Weir is trying to become the first Canadian to win his national championship since Pat Fletcher titled at this event in 1954. The best recent finish by a Canadian here is a tie for fourth in 1988, which was done by Dave Barr.
'Hopefully, the golfing gods will be here tomorrow,' said Weir of his quest for history.
Singh, who climbed into the No. 1 spot in the world with his win at the Deutsche Bank Championship last week, had a tough day. He missed the green at the first and fourth and was unable to save par.
The Fijian three-putted also for bogey at the second. He got one shot back with a birdie at the fifth, but stumbled to a bogey at the seventh. Singh fought right back to birdie the eighth to get back to 6 under.
Singh dropped in a birdie at the 10th, but trouble loomed. His tee shot at the 11th found the left trees. Singh, the 2000 Masters champion, tried to play over the trees, but his second came up short of the green and in water.
That led to a triple bogey that dropped him back to minus-4. Singh, who is battling a sore left knee, responded in fine fashion. He rolled off three straight birdies from the 13th to get back into a share of second. Singh parred his final three holes to remain there.
Parnevik, playing alongside Weir and Singh, struggled like Singh. The Swede lost his tee shot way right at the first. The ball was never found in the corporate tents, so he was given a free drop. No matter, he still stumbled to a bogey and slipped to another bogey at the fourth before posting a birdie at the fifth. Another bogey at the 11th dropped Parnevik to minus-5.
A five-time winner on the PGA Tour, Parnevik dropped in birdies at the 13th and 15th to return to minus-7. He bogeyed the 16th, but erased that error with a birdie at the last.
'It was a very different day to say the least. I started on one and being the professional I am, I stood over the ball and had a little bit of a slippery right hand. I thought I'd swing a little bit smoother, and I hit it 80 yards right,' said Parnevik, whose ball was later found in a fairway bunker on the fourth hole, which is adjacent to the tents he hit into.
'I bogeyed that hole, then started horrendously on the front nine which carried over to the other guys because they putted poorly, as well. It was a very different day. None of us could do anything right it seemed like.'
Kresge posted one of the best rounds of the day. He converted back-to-back birdies from the second before carding nine straight pars. Kresge picked up his third birdie at the 13th to move to minus-6.
Kresge, a two-time winner on the Nationwide Tour, dropped a shot at the 16th. However, he closed with birdies at 17 and 18 to grab a share of second place.
American Ryder Cupper Stewart Cink posted a 2-under 69 to move to 4-under-par 209. He shares seventh place with David Sutherland, Glen Day, Bill Haas and Mark Hensby. Steve Lowery, Dean Wilson and Hunter Mahan are one stroke further back at minus-3.