Portugal had already played their last three games without Cristiano Ronaldo, but with the most-capped player in men's international football hanging up his boots after 234 caps and a record 146 goals, it was officially a new era for the national team.
A 100% record heading into this match suggested Portugal would be fine, but it was Denmark who started the brighter of the two sides, with Ruben Dias required to make a potentially goal-saving challenge to deny Mohamed Daramy from a promising position.
Bruno Fernandes then lost Alexander Bah’s run in behind, but a square ball was crucially cut out by Renato Veiga.
Both sides struggled for clear-cut chances in the first period, yet as half-time approached, Denmark could have been ahead when Rasmus Hojlund turned in from close range, only for Mikkel Damsgaard to have strayed offside in the build-up.
However, Hojlund soon had more luck when his header led to a Dias handball, allowing Pierre-Emile Hojbjerg to net his 12th international goal after VAR intervention.
The team that opened the scoring in each of Denmark’s last seven competitive matches had gone on to win, and that prospect was looking likely once more after 50 minutes thanks to a second Danish goal.
Involved once more, Hojlund peeled off towards the back post to volley home Damsgaard’s searching ball.
Needing a response, Ruben Neves tested Mads Hermansen with a long-range strike, but Portugal’s uninspiring performance was swiftly compounded when Hojlund netted his fifth goal in as many Nations League matches, shifting the ball onto his right and sending a rasping strike past Diogo Costa.
Remarkably, Denmark bagged a fourth through a firm Joachim Andersen header from a set piece, leaving the hosts humbled and needing a result against Wales to top the group.
With Portugal’s progression to the Nations League finals already assured, Bernardo Silva was alert to tuck in a rebound after fellow substitute Joao Neves saw his initial effort saved, yet that was merely a consolation.
Second-placed Denmark must now prepare for a potentially decisive fixture against Norway on Tuesday, armed with a three-point cushion heading into their final group game.