Brian O'Neill says Team USA took the foot off the gas in their first game in PyeongChang, but he expects more from this team moving forward. (0:41)

GANGNEUNG, South Korea -- Five practices were supposed to prepare the Americans for this, and they knew at the second intermission that Slovenia was going to come out punching.

The preparation and the knowledge were not enough to fend off the fatigue, as the United States blew a two-goal lead and lost a 3-2 stunner in overtime Wednesday night in the Olympic opener for both teams. Slovenia captain Jan Mursak scored the tying goal with 1:37 left in regulation and the winner 38 seconds into overtime.

Long before that, the U.S. started letting the game slip away with mistakes all over the ice.

"We started turning the puck over in our zone, and they were getting chances, and that led to some momentum for them," said goaltender Ryan Zapolski, who allowed three goals on 25 shots. "We can't give up those chances that we were giving up out of nothing there in the third. That really kills your momentum. And they scored a goal off one of them, and from that point we were kind of on our heels."

Brian O'Neill and Jordan Greenway, the latter who became the first African-American hockey player for the U.S. at an Olympics, scored to build the 2-0 lead in a dominant showing, and the shots were 24-12 after two periods. Coach Tony Granato pointed out that Slovenia probably should have been the more tired team from playing so much in its defensive zone, but there was none of that from a group that has only one player -- Mursak -- with NHL experience.

In the game because of goalie Gasper Kroselj, who stopped 34 of 36 shots, Slovenia came to life when Jan Urbas scored 5:37 into the third period. With fans chanting "SLO-VE-NI-A," the perennial underdogs started pouring it on.

"We outskated them in the third, especially, and had more energy," said Mursak, who spent time with the Detroit Red Wings. "After we scored that first goal, I think we really got that extra energy and the feeling that can score some more."

Slovenia's Jan Mursak scored goals with 1:37 remaining in regulation and again 38 seconds into overtime, beating U.S. goaltender Ryan Zapolski. AP Photo/Frank Franklin II

After flashing the breakneck speed of forward Garrett Roe on O'Neill's goal and the quick reaction of Greenway on his rebound tally, the U.S. suddenly looked gassed. Granato wondered whether 21 players dressing in their first Olympic game combined with the hype and long day before a late start took a toll on his team, which hadn't played together much.

"Our energy in the third wasn't great," Granato said. "It could've been a little fatigue just set in mentally because of the way that the day was. But no excuses. ... They were the better team in the third, and it was good enough for them to get the win.

The U.S. at least picked up a point by getting to overtime, while the Russians lost 3-2 in regulation to Slovakia across town at Gangneung Hockey Centre. After each team's first game, Slovakia is atop Group B, followed by Slovenia, the U.S. and the Russians.

No one expected this kind of showing from Slovenia, which looked overmatched in the first two periods and is mostly known for having a star in Los Angeles Kings captain Anze Kopitar. There was no Kopitar -- this is the first Olympics since 1994 without NHL players -- but still the same pluck and no-quit attitude that helped the Slovenians beat rival Slovakia in Sochi for one of the biggest victories in national history.

Slovenia added another by upsetting the U.S. and will face the Russians next.

"I'm proud of my guys," Mursak said. "I'm proud of the team, and hopefully we can start looking to the next game and surprise somebody else."

The Americans didn't expect to be surprised by much but go back to the drawing board to prepare to face Slovakia on Friday.

"We'll learn from this," O'Neill said. "We haven't played a whole lot of hockey as a team together. So it's good for us to learn from that third period."