The Americans lived up to their favourites tag with a resounding win in Group A.
Alex Carpenter put the US ahead in the 16th minute of the first period with a power play goal, redirecting Megan Keller's shot.
Joy Dunne doubled the lead in the fourth minute of the second period, converting Tessa Janecke's perfect assist.
The American fans were back on their feet celebrating just over a minute later, when Hayley Scamurra slipped her shot through the legs of the Czech defender.
Following a successful penalty kill, the Czech Republic pulled one back after Barbora Jurickova returned from the penalty box to immediately score from close range in the ninth minute of the second period.
US captain Hilary Knight, in her fifth Olympics, restored her side's three-goal cushion two minutes from the end of the second period, and Scamurra netted her second in the fifth minute of the final period.
Sweden soar
Lina Ljungblom scored twice to get Sweden off to a perfect start in the earlier Group B game at the Rho Arena.
The 5,800-capacity venue had plenty of empty seats, but the Swedish and German fans brought the noise and colour. After all the concern over ice conditions and whether the arenas would be ready in time, the first puck drop of Milano Cortina arrived.
Germany, making their first appearance since 2014, more than held their own in an opening period where they were outshot 14 to six and took the lead after eight and a half minutes with a power play goal.
Katarina Jobst-Smith sent a powerful slapshot from just inside the blue line which sailed past the Swedish goaltender and ricocheted off the post into the roof of the net.
Sweden failed to take advantage of two power plays, coming either side of Germany's goal, but equalised in the 12th minute after a scramble in the crease where Ljungblom forced the puck over the line and they never looked back.
"It's my first Olympic goal so that's huge and my first goal for the season too," Ljungblom told reporters. "So obviously I'm really happy about it. I almost forgot how it is to score."
Ljungblom netted again in the eighth minute of the second period on a power play and Sweden made another power play count late in the period, with Mira Jungaker scoring in the final minute.
Sweden's Thea Johansson wrapped up the victory in the 12th minute of the final period, getting on the end of an impressive assist from out wide by Hilda Svensson.
Sweden are seeking a third podium finish but their last medal dates back 20 years when Italy also hosted the Games and the Swedes upset the Canada-US dominance of the Olympic finals by taking silver behind the Canadians.
The Swedes and Germany are favourites to emerge from Group B, but the Swedes will now fancy their chances of topping the standings.
"We wanted to win this game, especially against Germany. We know it's a tough team to play against," Ljungblom added.
"We want to win this group, it's a good start for us."
Historic start for the hosts
Tournament underdogs Italy face long odds in a competitive field but earned pride in a lop-sided win in the Group B game across town at Santagiulia arena.
Organisers had scrambled to get the built-from-scratch arena ready in time, but any remaining concerns evaporated as enthusiastic fans piled into the 14,700-capacity venue.
France's Gabrielle de Serres started the scoring with a power play goal in the 10th minute.
Italy replied quickly through Kayla Tutino, and forward Rebecca Roccella put them ahead with five minutes left in the second period.
The crowd at Santagiulia went wild as Matilde Fantin pounced quickly to find the back of the net 14 seconds into the third period and Kristin Della Rovere put the game completely out of reach less than five minutes later.
