Vanessa Hayden-Johnson and Daliborka Jokic each had 12 points for the Italian team, which trailed 44-37 early in the second half.
That's when the wheels came off spectacularly for Riga as Umana Reyer scored 20 unanswered points for a 57-44 lead with two minutes to go in the third quarter.
Riga battled back and twice closed the gap to just two in the final quarter but they never made it all the way and crashed to a third successive setback in the competition.
Simona Ballardini and Eva Giauro each poured in 10 points for Umana Reyer, while Mery Andrade and Aleksandra Vujovic scored eight apiece.
Umana Reyer were excellent at the free throw line, making 27 of 30 (90%).
Riga got a game-high 30 points from Essence Carson.