McNamara led the Orangemen (22-7) with a career-high 43 points on 11-of-16 shooting (9-for-13 from 3-point range), stepping up enormously with Hakim Warrick riding the pine in foul trouble.
McNamara's performance saved the Orangemen from what early on appeared to be a major upset in the works.
BYU (21-9) came out on fire, with Mike Hall hitting three 3-pointers to open the game and give the Cougars an early 9-7 lead. The game seesawed for the first eight minutes with both teams combining to hit 11 of the first 12 shots taken.
With the Orangemen collapsing down low, worried about BYU's NBA prospect, senior center Rafael Araujo, the Cougars unloaded a 3-point barrage, which McNamara answered each time, scoring 12 of Syracuse's first 21 points.
With the score tied at 21, BYU finally began getting the ball inside to Araujo to complement its 3-point onslaught. A 16-5 run led to the Cougars jumping out in front 37-26 with just 5:05 remaining in the first half.
In the middle of BYU's run, Warrick, the team's leading scorer, picked up his third foul, leaving Syracuse with only one offensive option and a 2-3 zone that was being picked apart.
"Once Hakim went out, I knew I had to step it up,"McNamara said. "I felt good and they were going in. I just had to keep throwing it up there."
With the Orangemen switching to an aggressive man-to-man defense, and McNamara continuing to throw it up there, Syracuse went on its own 15-6 run to close the half and tie things up at 42 heading into the lockerroom.
McNamara had 13 of Syracuse's final 15 points to close the half with 28 points on 7-of-8 shooting, 6-of-7 from 3-point range.
"Gerry was the only thing going for us in the first half," Syracuse coach Jim Boeheim said at the post-game press conference. "He put us on his back and carried us into the halftime tie."
Warrick awoke in the second half, scoring 16 of his 20 points, but the Orangemen were unable to pull away from the Cougars.
With Araujo, who scored 24 points and pulled down 12 boards, in foul trouble, the Orangemen opened up a nine point lead with 8:10 remaining, but BYU answered with a 13-3 run to take the lead 73-72 with 3:22 left to play.
Warrick responded with a lay-up and then McNamara hit his ninth 3-pointer of the afternoon to give the Orangemen a 77-73 lead.
McNamara hit 3-of-4 free throws in the final 17 seconds to secure the victory.
Freshmen Demetris Nichols added 10 points for the Orangemen, while Bigelow and Hall each had 17 for the Cougars.
After the game, Boeheim complimented his team's second-half defense, which held BYU to just 33.3 percent shooting, after the Cougars shot 59.3 percent in the first half against the zone.
"They hit us hard early," Boeheim said. "Our guys stuck to it and did a great job in the second half and we kept knocking down shots."
The Orangemen shot 55.8 percent for the game, including 11-of-16 (68.8 percent) from behind the arc.
The Orangemen will now take on Maryland (19-11), who narrowly escaped an upset at the hands of UTEP (24-7), 86-83, in a battle of the last two national champions.
McNamara Nets 43 As Orange Advance In Tourney
Cuse Nation Top Stories
Eric Dungey: Taking Orange To Next LevelSyracuse quarterback Eric Dungey returns to lead the Orange offense in 2017. The fearless field-general is looking to take the next step and "graduate" to the mastery stage head…
Cuse Nation7:15 PM
Commitment Calculator: Syracuse FootballCuseNation.com takes a look at the chances each of their targets picks the Orange to join their 2018 class.
Cuse NationYesterday at 6:15 AM
Commitment Calculator: Syracuse BasketballCuseNation.com calculates the likelihood that targets will commit to Syracuse in the 2018 cycle.
Cuse NationWednesday at 5:30 AM
Carrigan Excited About Syracuse OfferOne of the strongest performances during Peach Jam was from BABC big man Dimon Carrigan. He has been on the Syracuse radar for months, but picked up an offer recently.
Cuse NationSaturday at 7:15 PM
Peach Jam Day 3: Syracuse TargetsSee how Syracuse basketball recruiting targets faired on day three of the Peach Jam as pool play wrapped up.
Cuse NationSaturday at 5:28 AM