Proteas EKA AEL went on a decisive 15-2 run over a span of seven minutes in the second quarter and used that as a platform for victory over visiting Khimik.
Looking to make the EuroCup Final Four for the first time in their history, the Cypriot club won at home in Limassol 70-65 and will attempt to close out the best-of-three quarter-final series next Tuesday in Ukraine.
Khimik, who got 14 points apiece in the game from Wykeen Kelly and Sasa Bratic, encountered a slow-starting Proteas who trailed 15-6 late in the opening quarter.
Ryan Randle's tap-in closed the gap to 18-16 with 10 minutes gone.
After Olumuyiwa Famutimi made it 20-16 in favour of Khimik when play resumed, Duane Woodward sparked Proteas into life with a three-pointer.
Ali Bouziane scored down low to take his points haul to seven before captain Milutin Aleksic added back-to-back lay-ups.
Bratic stopped the bleeding with a made jump shot for Khimik, only for the hosts to score six more points in a row for a 31-22 lead.
The closest Khimik would get the rest of the way was four points.
That happened two minutes into the final quarter when Oleksandr Kolchenko drilled a three-ball to reduce Khimik's deficit to 52-48.
A killer blow then struck the Ukrainian team when Famutimi was whistled for a foul on Vasily Zavoruev, who was attempting a three-pointer, and he made all three of his shots from the charity stripe for a seven-point lead.
Kelly did give Khimik hope with an incredible four-point play midway through the quarter.
But the reliable Aleksic responded with a lay-up to take his points haul to 18 as Proteas stayed in front for the remainder of the game.
Aleksic had 21 points for the winners, who won despite being out-rebounded 42-31.
Duane Woodward also had 15 points and four assists in a winning effort.
Proteas did force Khimik to turn the ball over 18 times while losing possession themselves on just 11 occasions.