Rantanen had a chance to tie it with 46 seconds left when he tried to go five-hole from alone in the right circle, but Fleury made the save.
"I had a chance to redeem myself there. That was close," Fleury said. "He looked up like he was coming higher and squeaked it five-hole. Lucky enough to make a play on it."
It was the NHL's first full-capacity crowd this season at 17,504 fans.
"Once we got that second goal, momentum shifted," Vegas captain Mark Stone said. "That's the loudest I've heard a building in quite some time."
NOTES:The Avalanche are the 18th team Fleury has defeated in the Stanley Cup Playoffs, passing Dominik Hasek for second most in NHL history behind Patrick Roy (20). … Colorado failed to become the fourth team to win its first seven games of a postseason since 1987, when all series became best-of-7 (Pittsburgh Penguins, 2008; New York Rangers, 1994; Montreal Canadiens, 1989). ... Pacioretty's goal was the latest go-ahead goal in Golden Knights playoff history (57 games). … Golden Knights forward Ryan Reaves served the second of a two-game suspension from the NHL Department of Player Safety for roughing and unsportsmanlike conduct against Avalanche defenseman Ryan Graves in the third period of Game 1. … Avalanche forward Brandon Saad had his five-game goal streak end. … Rantanen extended his point streak to seven games (four goals, seven assists) this postseason. … Grubauer had won 10 consecutive playoff games, dating to last season. … Avalanche defenseman Cale Makar had an assist to extend his point streak to four games (one goal, six assists). … Each team had 50 hits, with Avalanche forward Gabriel Landeskog making a game-high seven.