Some players felt the World Cup semi-finalists might have got carried away after they went two goals ahead after only 17 minutes.
Certainly there was a point shortly after their second goal when Belgium were passing the ball around in midfield to cheers from their fans where they seemed to be tempting fate.
"Arrogance? No, we were surprised on counter-attacks, we forgot to defend well," coach Roberto Martinez said after the 5-2 defeat.
The Swiss, who lost 2-1 in Brussels, qualified for the semi-finals on their head-to-head record after both teams finished with nine points in Group A2.
"It's very difficult to give an explanation. We started well, then at a certain point, Switzerland had nothing to lose," added Martinez after losing only his second competitive match since taking charge following Euro 2016.
"We gave them opportunities to come back in the game. We must come out stronger, try to understand what we have done wrong. It is all the more painful that we had started very well."
Midfielder Thomas Meunier said Belgium had "everything in our own hands" after their flying start.
"It was a perfect game for us. We could have scored even more," he said.
"I think we just thought we reached the final four already. Tonight was the worst match with the Red Devils in my career."
Goalkeeper Thibaut Courtois agreed that the team thought they were home and dry.
"I think after our 2-0 lead the feeling went into our team, that this cannot go wrong," he said. "But this is a high-level competitive match and hopefully not reaching the final four will teach us a lesson for the future."
Midfielder Thorgan Hazard, scorer of both Belgian goals, described the game as bizarre.
"Maybe our start was just too good, we became too self-assured about our task," he said, echoing his team mates' comments.
"With a 2-0 lead we should never have given it away any more. We have to learn from our mistakes as today we learned a very important lesson."
(Writing by Brian Homewood, editing by Nick Mulvenney)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)