Jantel Lavender scored 25 points and 90, at Staples Center.
Noelle Quinn scored 19 points on 7-for-12 shooting and Natasha Lacy added 13 points and six rebounds. Tina Thompson and DeLisha Milton-Jones each contributed 11 points for the Sparks, who have won two in a row.
Penny Taylor scored a game-high 29 points and Diana Taurasi added 21 points for Phoenix, which is 4-6 over its last 10 contests.
The Mercury led by as many as 16, but held just a 62-60 advantage after three quarters, and the Sparks forced overtime after a strong fourth quarter.
In the overtime period, Los Angeles took its first lead of the game on a bank shot by Thompson to give the Sparks an 82-80 advantage. The Sparks stayed in front and got four key free throws from Milton-Jones in the final minute.