The tournament of upsets continues. Slovakia tallied three second-period goals in a span of 3:19 en route to an unexpected 6-3 win over the Americans in Wednesday's lone Group B game in Moncton.

"I think we played great," said Slovak goalie Adam Gajan, who made 33 saves in his World Junior debut. "It wasn't like the USA was the better team. So I think it was an even game. We just have to play like this, and we can win against any team, I think."

Libor Nemec, Dalibor Dvorsky, Robert Baco, Filip Mesar, Peter Repcik, and Alex Ciernik scored for Slovakia. Captain Simon Nemec racked up three assists.

The U.S. saw its power play click on Wednesday for the first time at the 2023 World Juniors. The Americans scored twice with the man advantage and once right after a Slovak penalty had expired. But that was scant consolation in light of their sloppy defensive play.

Tyler Boucher had a pair of goals and Gavin Brindley also scored for the Americans. Cutter Gauthier added two assists.

"It was emotional for sure," said Boucher. "You know, obviously we're a group that doesn't like to lose, and [Slovakia] came out and played a good game. Credit to them. Penalty trouble can get you into trouble in this tournament."

The U.S. program has been riding high recently with three of its five World Junior gold medals in the last 10 years (2013, 2017, 2021). But they'll need a much better, more disciplined effort to return to the final this year after an underwhelming fifth-place finish at the 2022 World Juniors.

"I think we just got away from our game in the second period and allowed a couple goals when we shouldn't have," said captain Luke Hughes. "I think we've just got to tighten up defensively and get back to our game. That's kind of what cost us."

A score that loomed large heading into this matchup was 5-2. The U.S. downed a feisty Latvian squad 5-2 in their opener. Slovakia, meanwhile, was hoping to rebound from a 5-2 loss to 2022 silver medalist Finland.

"It was a different game because Finland are playing a lot of defence, and that's not good for us," said Simon Nemec. "But today, we played well. And I think sometimes we were better than the USA."

In net, the U.S.'s Kaidan Mbereko, the 2022 World Junior starter, also saw his first tournament action. The U.S. outshot Slovakia 36-26, and Gajan was stellar and acrobatic.

Slovakia came out storming and took a quick 1-0 lead at 2:14. Libor Nemec cut around defenceman Ryan Chesley at the blue line, busted in off the left side, and banged his own rebound through Mbereko. The Slovak assistant captain is no relation to blueliner Simon Nemec, the #2 overall pick of the New Jersey Devils in 2022.

The U.S. struck right back with a power play goal at 4:42. Gauthier centered the puck to Rutger McGroarty, who directed it on net, and Boucher partially fell on Gajan while swiping the rebound in from his knees.

Slovak coach Ivan Fenes challenged the goal on the grounds of goalie interference, but after a video review, it was allowed to stand. Slovakia got a delay of game minor as a consequence.

The U.S. grabbed a 2-1 lead just after the Slovak penalty expired. Brindley deftly tipped Lane Hutson’s shot from the left side over a surprised Gajan’s right shoulder.

In the second period, things temporarily settled into a defensive groove. Mbereko made a fine save around the eight-minute mark when Slovak assistant captain Filip Mesar, a 2022 Montreal first-rounder (26th overall), set up Servac Petrovsky in front off the rush.

Then the tide turned in Slovakia's favour, suddenly and without warning.

At the midway mark, Dvorsky made it 2-2 with the man advantage. The 17-year-old AIK attacker, a top prospect for the 2023 NHL Draft, stepped in from the centre point and sent a magnificent wrister sailing past Mbereko's blocker side. The Slovaks had new life.

"Me and Maxim Strbak, we switched there a little," Dvorsky. "He passed to me and I faked a shot and then I shot and scored. That's obviously great, but I'm extremely happy that we won."

Next up, Baco streaked down right wing around U.S. blueliner Jack Peart and zapped a shot from the right faceoff circle home at 11:48. The heaviest player on Slovakia's roster at 100 kg, Baco slid hard into the end boards, but was soon up celebrating with his teammates at the bench.

The Slovaks kept on coming and the U.S. defence continued to struggle.

With a delayed penalty to the Americans and an extra Slovak attacker on the ice, Mesar made it 4-2 at 13:20 when his shot tipped off the outstretched stick of Charlie Stramel. The Avenir Centre crowd cheered lustily for the underdogs, who were having their way with Canada's cross-border rivals.

With under four minutes left in the middle frame, Logan Cooley embarked on a spectacular solo dash, but his attempt at a Peter Forsberg-style one-handed goal slid wide.

Poor discipline hurt the U.S.'s hopes of coming back. At 17:34, Kenny Connors threw a late hit on Slovakia's David Natny in front of the U.S. bench and was ejected with a five-minute major and game misconduct for boarding.

After the five-minute penalty expired in the third period, it was Gajan's time to dazzle with his green gloves and pads from the NAHL's Green Bay Gamblers. He robbed McGroarty and Chaz Lucius on excellent opportunities. At the other end, Mbereko stoned Alex Sotek on back-to-back chances and Adam Zlnka in all alone.

"We could expect that I will have more shots in the last period," Gajan said. "So I was a little bit tired. But the guys helped me, so they made it easy for me."

At 10:46, Repcik potted his team-leading third goal of the tournament on a partial breakaway. He fought off Luke Hughes' backcheck to dangle his way in, and the puck ended up going in off Hughes.

At 15:08, Boucher got his second of the night with a power-play rebound to cut the deficit to 5-3. But the Americans would get no closer despite pulling Mbereko for the extra attacker. Ciernik put the icing on the cake with an empty-netter at 18:53.

Frustrations boiled over just before the buzzer with a scrum in the Slovak end. After a lengthy review, during which Fenes' team stood with arms linked on their bench, Baco was assessed a five-minute major for cross-checking. The Slovaks celebrated wildly at game's end.

"After this game, you can't be tired!" said Simon Nemec, who played a game-high 25:31. "A lot of good things in this game."

With just two bronze medals (1999, 2015) to its credit all-time, Slovakia is hungry to keep its momentum going. This victory isn't quite on par with upsetting the Americans 5-3 in the 2009 quarter-finals in Ottawa on goalie Jaroslav Janus's 44 saves, but it's still an important building block.

This was just the fifth time Slovakia has beaten the U.S. in 21 World Junior meetings. The Americans have 15 wins and one tie versus Slovakia.

The U.S. takes on Switzerland on Thursday, while Slovakia battles Latvia on Friday.

"I think we're gonna have a bounceback here, and we're hungry," said Boucher.

Sotek made his World Junior debut for Slovakia with one assist after being added to the roster on Wednesday. The 18-year-old winger from Poprad has a goal and two assists in 21 games for HC Slovan Bratislava this season.