The Silver Stars took a 59-58 lead when Marie Ferdinand hit a 3-pointer with 7 seconds left. Ferdinand scored a game-high 21 points, but the Silver Stars (7-23) lost their fifth straight game for the third time this season.
Riley's game-winner came when she rebounded Katie Smith's missed shot on a drive into the lane with 3 seconds left. The Shock lead the league in rebounding, but were outworked, 39-33, by San Antonio.
Detroit's Cheryl Ford, leading the WNBA with 10 rebounds per game, was limited to three.
Trailing by 12 with just under seven minutes left in the first half, Detroit cut the deficit to 29-23 by halftime. The Shock tied the game for the first time on Smith's 3-pointer with 16 minutes left in the game.
Detroit is 5-2 since obtaining Smith in a trade July 31 from Minnesota, and has won three straight since with her in the starting lineup.
Ferdinand didn't score until 5:19 into the game. But as soon as she did, the Silver Stars went on a 19-8 run over the next 6:58 that turned a 5-4 lead into a 24-12 advantage. Ferdinand was outscoring Detroit, 13-12, at that time.
Penny Taylor led five players in double figures with 23 points, and the Phoenix Mercury snapped the Sacramento Monarchs' seven-game winning streak, 76-62, Friday night at Phoenix.
Phoenix's leading scorer, former UConn star Diana Taurasi, bruised her right knee diving for a missed shot on the final play of the first half and did not play in the second half.
In other games, New York stung Charlotte, 82-74, and Seattle dropped Minnesota, 72-66.
At Charlotte, N.C., Shameka Christon scored 7 of her 18 points in the second overtime, and the Liberty overcame an 18-point first-half deficit to beat the league-worst Sting.
At Minneapolis, Sue Bird scored a season high-tying 20 to lead Seattle. Lauren Jackson had 17 points and 10 rebounds for Seattle.
