Coach Gerard Gallant said he pulled Fleury because of the Golden Knights' performance, not his goaltender's. Fleury was pulled twice last season.
"I thought [the Flyers] played really well," Gallant said. "They took advantage of opportunities. I thought we played too soft. We talked about winning those blue-paint battles, and we didn't do it."
Jonathan Marchessault scored at 6:37 of the first period to give the Golden Knights a 1-0 lead. He took advantage of a Flyers line change, and defenseman Jon Merrill found him open along the left side for a high shot past Elliott.
At 3:30 of the first period, Golden Knights defenseman Shea Theodore's shot attempt hit each goal post and stayed out. Gallant said the game could've been different if it went in.
Instead, Oskar Lindblom scored on a 2-on-1 at 11:24 of the first period to tie it 1-1.
Simmonds scored his first at 18:43 on a deflection of a Hagg shot to give the Flyers a 2-1 lead.
"Tonight was a team effort," Simmonds said. "We played well. I thought we did a good job after [the first 10 minutes]."
Philadelphia scored three goals in the second period to take control of the game.
Simmonds got his second at 2:58 on the power play for a 3-1 lead, Hagg scored at 6:11 to make it 4-1, and Scott Laughton beat Fleury at 7:17 off a deflection for a 5-1 lead.
Pierre-Edouard Bellemare scored shorthanded at 8:28 of the third period to get the Golden Knights to within 5-2.
"At one point, we stopped skating," said Bellemare, who played for the Flyers from 2014-17. "It doesn't matter who you play in the League, you will pay the price."
The Flyers won their third straight season opener after losing their first under coach Dave Hakstol. Each of the three wins was on the road against a Pacific Division team. Simmonds had a hat trick in the 2017-18 opener against the San Jose Sharks.
"I think we skated better (than the Golden Knights), to be honest," Voracek said. "We didn't give them much time to do something with the puck and we did it for a full 60 minutes."
