Washington (7-4) has won four of its past five games and six of its past eight.
Chicago (5-7) has lost four games in a row, the franchise's longest losing streak since a seven-game skid early in the 2001 season, when the Sky responded with seven straight wins and eventually won the WNBA title.
Delle Donne made a jumper and hit two free throws before Sykes found Shakira Austin for a basket that made it 6-0 and Washington never trailed. Sykes had 14 points on perfect shooting in the first half, going 5 of 5 from the field, including a 3-pointer, and 3 of 3 from the free-throw line.
Courtney Williams made a driving layup with about a minute left in the first half that trimmed Chicago's deficit to 39-38, but Delle Donne answered with a three-point play to give the Mystics a four-point halftime lead and Austin scored five points in a ,12-0 run to open the third quarter that made it 54-38 with seven minutes left in the period and the Sky trailed by at least eight points the rest of the way.
Austin finished with 12 points.
Rookie Morgan Bertsch, who had missed the past eight games because of an ankle injury, shot 6 of 9 from the field, 4 of 5 from 3-point range, and scored 16 points with a career-high-tying seven rebounds for the Sky. A third-round pick by the Dallas Wings in the 2019 draft, Bertsch made her WNBA debut in Chicago's season opener and started the Sky's first three games (scoring 16 total points) before she suffered an ankle injury in an 81-70 loss to the Mystics on May 26.
Marina Mabrey added 11 points for Chicago. Alanna Smith added eight points and grabbed a career-high 13 rebounds.
The Mystics shot just 39% (29 of 64) from the field and made 5 of 15 from 3-point range but hit 22 of 26 from the free-throw line and committed just five turnovers. Chicago hit 27 of 61 (44%) from the field but shot 9 of 14 from the foul line with 20 turnovers, which led to 26 Washington points.