Switzerland took their first win in the tournament, and with it, an important mental hold over Japan, as the teams will meet again in the relegation round.
Alessia Baechler and Alena Rossel scored for Switzerland, Harua Umemori for Japan.
"It was a tough game, but fortunately, we managed to win," Rossel said.
The game was close, as expected.
Japan has played Switzerland at this level more than any other team. This makes the 11th all-time meeting between the teams. Switzerland is now 10-1. Five of those games have been one-goal games. Two were decided in a shootout.
On a delayed penalty at 4:07 of the first period, the Swiss maintained possession for several seconds. The puck ended up with Alessia Baechler who relied on a screen in front and scored on a slap shot from the top of the right circle.
Switzerland nearly double the lead a few minutes later when Naemi Herzig blocked a shot at the Swiss blue line and skated in on a breakaway, but hit the crossbar.
At 3:51 of the second period Switzerland slap shot from right point deflected in front with a high stick by Annic Buchi. It was ruled a goal on the ice, but overturned following video review.
Both teams seemed to have the relegation round already in mind. For example, both teams made a goalie change halfway through the game.
Switzerland doubled their lead seven minutes into the third period when Alena Rossel's shot from the blueline somehow found its way to the back of the net.
"I took a shot and didn't realize it went in, until I saw my teammates celebrate," Rossel said.
But Japan showed heart all the way til the end and they got theior reward when Harua Umemori scored Japan's lone goal with just six seconds remaining in the game.
The teams will play again on Thursday.
"Now we know how they play, but they also know us so we'll see. We have to play for each other, as a team," said Rossel.
The table is set for games number 12 and 13 between Japan and Switzerland U18 teams.
The stakes are high.