Elena Delle Donne scored 22 points and the Chicago Sky beat Washington 93-79 on Sunday to prevent the Mystics from clinching a playoff spot.
Epiphanny Prince added 21 points for the Sky (22-9), who have already clinched the Eastern Conference's top playoff spot and finished the five-game season series with four victories over the Mystics.
Swin Cash had 16 points and Sylva Fowles had 13 points and nine rebounds as Chicago won its seventh game in its last eight.
Ivory Latta scored 18 points on six 3-pointers for Washington, which closed within two points in the third quarter but never led after the first. The Mystics have lost two straight.
Crystal Langhorne added 16 points for the Mystics, who can still clinch their first playoff berth in three seasons with a win or a loss by New York. Washington visits Indiana on Tuesday night, while New York hosts Phoenix.