After receiving a 20-0 victory following a forfeit by CSKA Moscow, Riga followed that up on Wednesday night with a 61-57 upset of previously unbeaten Umana Reyer Venezia in Latvia.
In an extremely tight game, Umana Reyer played as if they were going to escape hostile territory again with another win but Riga held their nerve and made big plays down the stretch for their third victory in eight games.
Three times in the fourth quarter Umana Reyer went in front and each time Riga pegged them back.
After Vanessa Hayden-Johnson split a pair of free throws with 4:04 remaining to knot the game at 52-52, Essence Carson put Riga in front again with a jump shot.
That proved to be the go-ahead basket, although Riga had to maintain their concentration until the very end.
Jenifer Nadalin drilled a jumper to trim Riga's advantage to 59-57 with just under a minute to play and Riga boss Ainars Cukste elected to call a timeout.
When play resumed, Carson missed a jumper but got the offensive rebound, only to have the ball stolen by Mery Andrade of Umana Reyer.
Italy international Simona Ballardini was called for stepping out of bounds with 24 seconds remaining, however, turning the ball back over to Riga.
The visitors put Riga's Carson on the line and she made two for the four-point lead.
Ballardini attempted a three-pointer but missed and Umana Reyer lost for the first time in their excellent EuroLeague Women campaign in Group B.
Riga captain Ieva Kublina had 12 rebounds to go with her 10 points.
At 7-1, Umana Reyer have already clinched first place in Group B.
Though CSKA Moscow can still win seven games, their failure to show up for the Riga game last week due to their temporary suspension of operations cost them a point in the standings.
No other side can reach seven wins in the group.
Nadalin had 14 points for Umana Reyer.