During the second half, Sporting seemed on the way towards securing a comfortable win in the first leg between the two clubs. Sporting was ahead by five goals on several occasions and attacked for a six-goal lead, but in the end, Aalborg fought back.
In the last minute of the match, Sporting's Portuguese international Francisco Costa brought Sporting ahead by one with his 12th goal of the match.
But in the last seconds, Mads Hoxer burst through the Sporting defense to earn a penalty throw, and Buster Juul, who had not yet been on the court, made no mistake from seven meters.
So everything is completely open before the return leg takes place next Wednesday in Gigantium for a place in the Champions League final weekend in June in Cologne.
Things looked promising for the Danes from the start in the Portuguese capital as Aalborg looked well-organized and quickly took a 7-4 lead.
However, the Danes really struggled to contain the threat of Francisco Costa, and after fifteen minutes, he had scored four goals to limit the Sporting deficit to one.
The Danes still seemed in control, but poor finishing and three successive Portuguese goals meant that the two teams went into the break level on goals.
When Aalborg lost to Sporting in the group stage in February, the North Jutland team was in control in the first half, but then the Portuguese came out storming in the second half. And that was also exactly what happened on Wednesday.
With six unanswered Sporting goals, Aalborg suddenly found themselves trailing 17-22, and that prompted coach Simon Dahl to call a timeout.
The Danes subsequently looked more composed in attack, with Hoxer taking responsibility at right-back. With a few good actions, he helped Aalborg shave off some of the Portuguese lead.
There were opportunities for Aalborg to regain control of the match, but both Arnoldsen and Juri Knorr missed free throws. With Francisco Costa doing his magic at the other end, Aalborg had difficulty closing the gap.
For a long period, Aalborg trailed with a couple of goals, but as the game neared its conclusion, Fabian Norsten in the Aalborg goal pulled off a couple of great saves.
This time, Aalborg seized the opportunity, and with five minutes remaining, Arnoldsen equalized with a goal into an empty net, ensuring a very close finish.