Gor Mahia, who won the trophy last season after beating Ulinzi Stars 2-0 in the final, were hosting the brewers, meaning the latter go into the second leg with an advantage of a win and an away goal.
Ugandan important Robert Omunuk made all the difference rising above Gor Mahia's back four to head home past Tanzanian keeper Ivo Mapunda in the 40th minute.
The match saw a fiery start with both sides eager to make their chances count early on. As early as the third minute, Jesse Were had already tested Ivo Mapunda but the Tanzanian national, did well to save a shot from the former Mathare man.
Gor were relying on counter attacks in the opening exchanges with Tusker having started faster in attack, but their execution was always wanting. Abbey Natty, who had been given a rare starting role, did well to win the balls but what he did with the ball always pulled down all his hard work.
Gor were forced into an early sub as Antony Gathu left injured after being hacked down with Anthony 'Teddy' Akumu taking his place in midfield.
Andrew Tololwa had another shot at Mapunda from almost 30 yards out, but he was not lucky with a fully stretched Mapunda saving off the ball for a corner. Joseph Shikokoti headed over the resulting corner from Andrew Tololwa.
However, a resilient Tusker finally found the way into the net in the 40th minute. Ugandan Robert Omunuk, definitely not the tallest of players inside the box managed to snitch in a header from Andrew Tololwa’s low corner.
The brewers got into the second period more calm and composed despite Logarusic bringing in Edwin Lavatsa for Abbey Natty and Moses Odhiambo for Juma Yussuf. Gor struggled to break Tusker's backline with their best chance coming in the 79th minute, Kelvin Omondi’s shot going narrowly wide.
Ali Abondo also had a late chance but his shot was well handled by Samwel Odhiambo. Gor will now need to score at least two goals without conceding to advance to the finals after Tusker scored an important away goal.
The winner of return match will meet either Chemelil Sugar or Thika United in the final later next month.
Chemelil inched closer to a first final in the knockout competition thanks to late 1-0 win over Thika United in the first semi-final played last week.