Griner's reverse layup capped Phoenix's 12-3 run at the start of the third quarter that made it 48-31 lead. Washington went 4-plus minutes without a field goal and Phoenix extended its lead to 53-33 on Leilani Mitchell's corner 3-pointer. Phoenix outscored Washington 21-7 in the decisive third, holding the Mystics to 2-of-10 shooting.
"We definitely let our defense shine tonight and our offense as well," Griner said. "If you're playing good defense, then your offense will come too."
Cayla Francis, Mistie Bass, Shameka Christon combined to score all 16 of Phoenix's fourth-quarter points. Phoenix (18-13) got 33 points from its bench players and had 21 assists on 31 field goals.
"Everybody got to play tonight, that's always a good thing," Dupree said. "It gave some of the players that play a lot of minutes some rest and the people that came off the bench did a great job for us."
Defending WNBA champion Phoenix trails Western Conference-leader Minnesota by 2 1/2 games and leads third-place Tulsa by 2.
Emma Meesseman and LaToya Sanders each scored 10 points for Washington (16-13). The Mystics made just 19 of 61 shots (31 percent) and their 53 points were a season-low for a Phoenix opponent.
Washington beat Phoenix by eight points on Aug. 28 in their only other matchup this season.
"They were more aggressive today," Washington coach Mike Thibault said. "I think they doubled our posts a little bit more today. I thought their defense was a little more active today than it was when we played them last week. That was probably the biggest part of it."
DeWanna Bonner, who ranks fifth in the league in scoring at 16.3 points per game for Phoenix, was held to just four points in 19 minutes.
Bonner's first field goal was a first-quarter buzzer beater to give Phoenix a 17-15 lead. Griner scored four straight points to cap Phoenix's 15-2 second-quarter run for a 34-21 lead. Washington made just three of its first 13 shots in the second and trailed 36-28 at halftime. The Mystics stayed close by grabbing seven offensive rebounds and forcing nine Phoenix turnovers.
Dupree made all six of her first-half field goals, including a jumper just before halftime to stop Washington's 6-0 run.
"We had a little run before the end of the half that could have gotten us back in it and the start of the third quarter was crucial, they were good and we were bad," Thibault said.