Brno's 83-76 all but eliminated Riga from contention for the next phase.
Though both can still finish with 4-6 records depending on next week's results, Gambrinus would win the tie-breaker because of a superior goal differential in their games against the Latvians.
At one stage, play-off basketball looked as if it might disappear for Brno.
They led for much of the second half but Riga tied the contest at 73-73 with 5:16 left on an Essence Carson jump shot.
Brno responded in grand style.
Hamchetou Maiga Ba made a lay-up, Petra Kulichova split two at the free throw line and Hana Horakova went in for a lay-up to take her points haul to 20 for a 78-73 lead with 3:50 to play.
The Czech giants, finalists in last season's competition, pulled away for a seven-point triumph.
Horakova finished with 23 points while Tamika Whitmore scored 22 and pulled down 10 rebounds.
Eva Viteckova also had 20 points for Brno.
Carson scored 33 for Riga.
Vlastimil Havlík (Gambrinus Brno assistant coach):
"For both teams it was an important and nervous game. We had a part where we created fast breaks and made easy points. But we also had those slow parts, when only one player with ball was running but the others seemed to stand still. It was when Riga made their easy points. Riga had a very good player in Carson - she surprised us with her shooting from the perimetre. We are lucky to make it to the playoffs."
Ainars Cukste (Riga coach):
"I don´t know what happened with our team in the first three minutes. We played without movemen and rebounds and lost that part 0-14. But we had a very good motivation for this game. I am satisfied with the rest 37 minutes. But our bad start of the game was the deciding factor for our loss."