It was the 10th time the Canadian has had at least a share of the 54-hole lead on the PGA Tour, but he has only one victory when leading going into the final round.
Protecting this one might be the toughest yet.
Camilo Villegas ignored the swirling wind and increasingly firm conditions at TPC Boston to shoot 63, putting him in the final group with Weir for perhaps his best chance at his first PGA Tour victory.
Three shots behind were Sergio Garcia (68) and Vijay Singh (69), part of the playoff last week at The Barclays that Singh won to move atop the standings in the PGA Tour Playoffs for the FedEx Cup.
And right behind them were Jim Furyk, looking for his first victory in an otherwise solid year, Ernie Els and Ben Crane, whose 63 came during the morning before the wind hit full strength.
"I've got my hands full," said Weir, who was at 17-under 196.
Ten players were separated by five shots going into the Labor Day finish, the kind of shootout the Deutsche Bank Championship always seems to produce.
Tiger Woods isn't around to take part in this one, and neither is defending champion Phil Mickelson. He played an ordinary round of 1-over 72 and missed the 54-hole cut, leaving questions about whether he will play next week in St. Louis in the third round of these playoffs.
Weir has been around the week since opening with a 61, but he was equally pleased with a 67 considering he struggled to find fairways and hit only half the greens. But he got moving in the right direction with a 7-iron that stopped just under 3 feet away on the 14th, one of only eight birdies on the day.
Conditions were much more difficult, but Villegas and Crane sure didn't take notice.
"I thought a 66 or a 65 would be a great round," Garcia said. "I didn't see a 63."
Villegas relied on a tip from Singh — the power of positive thinking. Neither is regarded as a wizard with the putter, but Singh won last week at The Barclays after saying he would stop paying attention to negative comments about his short game and believe he was among the best.
"Starting this year, I decided to tell myself something similar to what Vijay told himself last week, that I'm a great putter and that I'm a lot better than people think and people write," Villegas said. "So today was a good reflection. And if you look at my putting stats for the year, I'm one of the great putters on tour."
Which tour he was talking about remains a question, however, as Villegas is not among the top 80. But he was brilliant in the third round, taking only 22 putts, including a birdie on the 14th that he thought he left short. Villegas immediately pulled up out of his stance and walked toward the cup, stopped, then shrugged his shoulders when it dropped.
Equally important was judging the wind, which seemed to blow sideways on every hole and was so tricky that players had a tough time deciding if it was hurting or helping their shots.
One such moment came at the 12th, with the pin tucked to the right over a hazard. Villegas hit a 9-iron that turned out perfect, caught the slope and stopped 12 feet away.
He believes he is ready to win, but his task is no easier than what Weir, Garcia, Singh, Els or anyone else faces. More wind is in the forecast, and the TPC Boston likely will play as tough as it has all week.
Villegas kept watching young players win — Anthony Kim, J.B. Holmes, Andres Romero, Sean O'Hair — and wondered why he couldn't join them.
"It's getting into me in a good way," Villegas said. "It's time to step it up and give a little kick on my butt and join them. So I've been working on that."
Garcia had one scary moment with a shot out of the thick rough on the 17th, in which the club stuck into a clump of grass. He said his wrist stung, but it didn't bother him on the 18th, even though he missed an easy birdie chance. The Spaniard would love another chance to win, coming off his runner-up finish at the PGA Championship and The Barclays.
Singh, meanwhile, has a chance to turn the second year of the playoffs into a snoozer.
He beat Garcia and Kevin Sutherland in a playoff at Ridgewood last week to move atop the standings, and if he can win the Deutsche Bank (he won in 2004 to become No. 1 in the world ranking), he would build at least a 9,000-point cushion after just two weeks.
Els feels as though he let a good year get away from him, winless since the Honda Classic. Furyk is one of only two players on the U.S. Ryder Cup team without a victory.
"There's great players on the leaderboard," Weir said. "Some guys that are hungry to win for the first time, guys that haven't won many times, and I'm trying to do the same thing. I just want to keep playing my type of golf. Hopefully, I can hit it a little better and find a few more fairways."