– The Minnesota Region of Death, as Air Force coach Frank Serratore calls it, entered Friday with three Minnesota teams. Now it’s down to one, and Serratore’s team is a big reason why.

Serratore’s Falcons upset St. Cloud State, the No. 1 overall seed in the NCAA men’s hockey tournament, 4-1 on Friday in the West Regional semifinals on Friday in front of 7,992 at the Denny Sanford Premier Center. Billy Christopoulos made 39 saves and Tyler Ledford scored two goals as Air Force advanced to Saturday night’s final against Minnesota Duluth, a 3-2 overtime winner over Minnesota State Mankato.

Serratore joked earlier in the week that the NCAA put his team in a regional with St. Cloud State, UMD and MSU Mankato to guarantee none of three Minnesota teams in the NCAA field would make it to the Frozen Four on April 5 and 7 in St. Paul. The Falcons (23-14-5) are one win from doing that.

“This group, they just refuse to surrender,’’ Serratore said. “… Collectively, they’ve got something special.’’

The victory was Air Force’s second over a No. 1-seeded team. In 2009, the Falcons knocked off No. 1 Michigan in the East Regional.

St. Cloud State (25-9-6) lost for the second time in three years as a No. 1 seed. Two years ago, the Huskies fell 5-4 in overtime to Ferris State in St. Paul.

“This is miserable,’’ St. Cloud State junior forward Robby Jackson said, fighting back tears. “I didn’t think I could ever hurt worse than we lost to Ferris a couple years ago, and here we are.’’

St. Cloud State outshot Air Force 40-26, but the Falcons blocked 29 of the Huskies’ 78 attempted shots. Christopoulos didn’t give up a goal until St. Cloud State’s Blake Lizotte scored with 2:51 left to cut it to 2-1.

“This kid is dialed in right now,’’ Huskies coach Bob Motzko said of Christopoulos, who has given up two goals in his past four games. “If he’s dialed-in again tomorrow, Air Force is going to win. Someone’s gotta score on this kid.’’

The Huskies tried to pull goalie David Hrenak for an extra attacker, but they lost possession of the puck while Hrenak was skating off, and Air Force’s Jordan Himley scored an empty-net goal with 1:36 left for a 3-1 lead. The Falcons added a second empty-net goal by Kyle Haak with 48 seconds left.

– its fourth OT triumph in the past three NCAA tournaments. UMD’s Nick Swaney appeared to score 55 seconds into OT, only to have the goal waved off because of goaltender interference after a video review.

“That was a little tough on the ticker,’’ said Bulldogs coach Scott Sandelin, whose team trailed 2-0 in the first period but held the MSU Mankato (29-10-1) to no shots on goal in the second. Swaney’s power-play goal with 4:08 left in the third tied it 2-2.

In an action-packed 2:28 of overtime, the Mavericks had a chance to win, too. But Nicholas Rivera’s shot into a seemingly open net instead hit teammate Brad McClure, who had been dumped into the net.

“We had a good start; everybody was excited and dialed in for the game,’’ said senior forward Zeb Knutson, the Sioux Falls native who scored 2:21 into the game for the Mavericks. “Somewhere along the way it got lost.’’