Aris hammered SIG Strasbourg 71-52 to put one foot in the Basketball Champions League last 16, while Ventspils grabbed a first-leg lead over Umana Reyer Venezia.
With home-court advantage, Aris made it count as Will Cummings contributed 21 points.
Strasbourg enjoyed a superior group-stage record to Aris, but they came up short on their travels, giving up 26 turnovers.
"Today we came out, we played hard and we hustled for every possession," Aris head coach Dimitris Priftis said.
"We had a good attitude that is expressed from our defensive desire and effort. I still don't believe we played good offensively."
Ventspils also won on their home court, their seven-point advantage at the final siren was their biggest lead of the game.
"It's the first half of a full game and we're going to go back and make our corrections and be prepared for the next game," Venezia forward Melvin Ejim said after the 74-67 win.
The second leg of each tie will be played on February 22.