The 14-year-old had four points -- a hat trick and an assist -- as Slovakia earned its second straight win, 4-1, over Switzerland.
Three of her points came in a six-minute stretch during the second period. The forward completed the hat trick just under four minutes into the third.
"I'm amazed." Lopusanova said through a translator. "I'm glad I could help out the team as much as I could. I can't really explain it in words. I'm just very happy."
With seven points (five goals, two assists) in two games, Lopusanova holds a share of the scoring lead alongside the United States' Margaret Scannell.
"I'm super lucky to have teammates who can pass to me," Lopusanova said. "I can score really easily off those passes.
"We're such an amazing group. You can hear it (now) in the locker room," she added, noting the chants and singing reverberating in the hallways at Östersund Arena. "Everyone is putting their teammates in a position they can score from."
She first squared the game at one apiece with a shorthanded goal at 1:53, weaving through three Swiss players before wristing the puck inside the left, near-side post.
A little more than two minutes later, Lopusanova fed Zuzana Dobiasova at the right edge of the crease. The Slovak captain lifted the puck past Swiss netminder Margaux Favre into the top, right corner to make it 2-1.
Lopusanova then extended the lead with her second of the game at 7:57.
That led to Swiss coach Melanie Haefliger to swap netminders, pulling Favre in favor of Talina Benderer. Favre had 11 saves in 27:57 of action. Benderer finished with 13.
At the other end of the ice, Slovakian netminder Livia Debnarova earned the win with an 18-save effort.
"That was an amazing game on her side," Slovakia defender Lily Stern said. "She really saved us a couple times."
Switzerland scored the game's opening goal at 8:56 of the first period when Alena Rossel picked off a shot from the blue line by defender Melissa Capezzali in the slot. She turned around and slid the puck under Debnarova's right pad.
Veteran defender Alessia Baechler received a game misconduct for checking from behind 6:10 into the third period. Although the Swiss defense prevented Slovakia from adding to the score, they had little opportunity to shift momentum in their favor.
The victory puts Slovakia quarterfinals for the second straight year. They currently sit atop Group B with six points and a plus-6 goal-differential -- one better than rival Czechia.
The only thing left to decide is seeding and bragging rights between Slovakia and Czechia, who as Czechoslovakia peacefully split into separate nations in 1993. The rivals meet Wednesday at 12:00.
"It's going to be a hard game, but we're well prepared and we're going to beat them," Lopusanova said.
If so, it is guaranteed to set off a post-game celebration even louder than the one which followed Monday's victory -- nd it will be within an earshot of the Czechia locker room which is next door.
Slovakia is 0-3 in the all-time series at the U18 Women's World Championship. The Czechs have outscored the Slovaks 16-3 in those meetings.
"You need to expect big things," Lopusanova said. "Just wait and see."
Switzerland will face Japan in its final round-robin contest Wednesday at 18:00. The teams will then move to the relegation round to determine who remains at the top level.
"First we have to calm down, think about what is going wrong," Haefliger said. "I'm sure we have to change something. The first thing is to get new energy, new motivation, a new goal, be positive. It's all we can do."
And for that, they can look to Slovakia. Last year, the Slovaks were on the verge of relegation before scoring exactly the number of goals they needed to advance to the quarterfinals.
Haefliger thinks last year's events can inspire her own team.
"We have players who saw that," she said. "We know that we have to keep going every game, stay positive."