A strong second half enabled the City Rise Bombers to clinch their spot with a 91-67 win over the Mid City Magic Lions, also confirming the Andy Bay Falcons' spot in the process.
The two sides will meet in the sudden-death minor semifinal on August 19.
The Mid City Magic A remained unbeaten at the top of the table, while its opponent in the majors semifinal, the St Kilda Saints, got back on track with a win.
Needing to win all their remaining games to stay alive, the Lions stuck with the Bombers for half the game.
It was 48-48 at halftime and the Lions led by double digits at one point in the second quarter.
The duo of Isaac Smiler and Jivarne Joseph proved difficult to shut down until the Bombers rallied late in the quarter.
They pulled in the deficit and an Olly Smith three-pointer tied the score at 46-46 just before the break.
From there the Bombers took control.
Steve Robinson found his way to the line twice to open the third quarter with four points, before hitting a three-pointer.
Anton Hughes then took the score to 57-50 three and a-half minutes into the quarter.
The Lions called a timeout at that point, but it was the key stretch in the game.
Robinson went on to score 11 of his 30 points in the quarter, helping the Bombers open a 73-61 lead at the final break.
At the other end, the scoring dried up for the Lions.
They managed just 19 second-half points, including only six in the fourth quarter.
Robinson was no doubt key for the Bombers, but it was a strong team performance as they showed their range of threats.
After a slow start to the season, they have now claimed their third consecutive win.
With plenty of talent on their roster, they are capable of knocking off anyone on their day and a genuine threat in the playoffs.
The Mid City Magic showed its class with a 101-69 win over the Andy Bay Falcons.
Benoit Hayman had 26 points, while five others reached double-figures, as the Magic killed the Falcons in transition and on the offensive glass.
In the late game, Matt Gillan hit six three-pointers and top-scored with 22 points as the St Kilda Saints beat Varsity 80-60.
