SALT LAKE CITY — At first, Bryant McIntosh thought he was the one who messed up.

The Northwestern guard was so surprised Vanderbilt’s Matthew Fisher-Davis fouled him intentionally that he initially assumed he had the score wrong.

“I was kind of shocked,” McIntosh said. “I thought maybe I had made a mistake. Maybe we were up one and they needed that foul.”

The scoreboard, of course, told a different story. It was Fisher-Davis who lost track of the score. Instead of Vanderbilt being one defensive stop away from securing a dramatic first-round NCAA tournament victory, Northwestern’s best free-throw shooter was going to the foul line with 15 seconds to go and a chance to put his team in front.

Fisher-Davis’ costly mental gaffe proved to be the biggest play in Northwestern’s first NCAA tournament victory, a 68-66 win that earns the eighth-seeded Wildcats a second-round crack at top-seeded Gonzaga on Saturday afternoon in the West Region. McIntosh calmly sank the tying and go-ahead free throws, Vanderbilt’s Riley LaChance missed a deep three on the ensuing possession and then the throngs of purple-clad fans at Vivint Arena could finally exhale.

As a sea of elated Northwestern fans exchanged hugs and high-fives all around him, the loneliest man in Vivint Arena hunched over at the waist and fought back tears. Fisher-Davis had scored 22 points and helped spearhead Vanderbilt’s comeback from a 15-point second-half deficit, yet he knew all too well that it was his ill-timed foul everyone would remember.

In a somber, tear-stained Vanderbilt locker room, Fisher-Davis took full responsibility for his mistake. He said nobody on the bench wrongly instructed him to foul, nor did he blame the coaching staff for using all of Vanderbilt’s timeouts prior to that sequence.

“I made a dumb-ass foul,” Fisher-Davis said. “That was it.

“Obviously it’s hard to take. I especially feel bad for our seniors going out like that off a play like that.”

A two-point first-round victory over 16-loss Vanderbilt might not be that big of a deal for most major-conference programs, but to Northwestern, it meant everything. It ensured a Wildcats program that had waited 78 interminable years to finally participate in March Madness wouldn’t have to wait any longer to experience an NCAA tournament victory.

Northwestern’s purple-clad faithful descended on Vivint Arena from places near and far for Thursday’s game, eager to see their basketball program finally make its big-stage debut.

There were celebrities and everyday Joes, alums and fans, old men and women who had waited their whole lives for this and grade-school kids too young to fully appreciate the significance. One family of six arrived clad in matching purple fedoras. Another group showed up wearing purple blazers and Donald Trump wigs. Countless more donned purple wigs or sunglasses and waived homemade banners or logo flags.

“The crowd was unbelievable,” McIntosh said. “When they made their run, the Northwestern fans stood up and gave us some energy. It was a tremendous help and we can’t thank them enough.”

It’s no surprise Northwestern had such a massive turnout Thursday because no other program in this year’s field has waited nearly as long for a taste of success. Not only were the Wildcats the only major-conference program never to make an NCAA tournament, only seven times have they even been good enough to reach the NIT.

Northwestern’s fortunes began to turn in 2013 when it plucked head coach Chris Collins off Duke’s staff. The son of longtime former Chicago Bulls coach Doug Collins quickly got to work selling his vision for an NCAA tournament-caliber program, landing a heralded first recruiting class that included McIntosh and fellow standouts Vic Law and Scottie Lindsey.

Progress didn’t come immediately. Northwestern lost 12 Big Ten games both of Collins’ first two seasons and failed to reach the NIT despite a 20-12 record last year. But the experience the Wildcats gained and the setbacks they endured paved the way for this magical 23-win season.

It was Northwestern’s veterans who helped Northwestern extend a seven-point halftime lead to 15 midway through the second half.

McIntosh finished with 25 points on 10-for-16 shooting, damaging the Commodores off the dribble and with his outside shooting. Guard Scottie Lindsey added 14 points and center Dererk Pardon also tallied 14 including some huge free throws down the stretch.

Vanderbilt never would have been able to storm back to take its first second-half lead in the final two minutes without Fisher-Davis, yet the 6-foot-5 junior found himself apologizing to his teammates in the locker room when the game was over. For the most part, his teammates were disappointed yet forgiving.

“He’s got nothing to apologize for,” LaChance said. “We’re not even close in that game without him.”