Griner scored 16 points, including the dunk during a key fourth-quarter stretch, as the Mercury extended their franchise-record winning streak to 11 games with a 90-78 win Tuesday night over the Washington Mystics.
''It's big, it's big,'' Griner said. ''I was in position, got the pass, went up. If your arm and ball are above the rim, you might as well dunk it.''
Griner, who added eight rebounds and two blocked shots, has four dunks in her year and a half in the league. No other WNBA player has ever had more than two.
After leading by double digits much of the night, Phoenix allowed Washington to get within eight before pulling away.
''You are talking about a team that is amongst the most talented teams in the history of this league,'' said veteran Mystics coach Mike Thibault. ''You have players who are among the elite. They blitzed us early, and we hung in the game. There are very few losses you feel good about as a coach but I feel like tonight we made some progress because we didn't back down from a great team.''
The Mercury (17-3) ran off six straight points after Washington had cut the deficit to 73-65 with 8:43 left in the game. Phoenix had led by double digits since the last three minutes of the first quarter.
''It's more about focus, I think they get a little too comfortable, things are getting a little too easy for us,'' Phoenix coach Sandy Brondello said. ''It's attention to detail. That's what has made us such a great team in our wins.''
Candice Dupree, who was announced as a WNBA All-Star reserve earlier Tuesday, had 19 points, six rebounds and four assists for Phoenix, which has won 13 of 14.
''We are playing really well as a unit,'' Dupree said. ''Third and fourth quarter we let up a little bit defensively. But overall I thought we played pretty well.''
Diana Taurasi added 18 points, five rebounds and four assists for the Mercury. No Phoenix team has ever been 14 games above .500. Phoenix was 23-10 at one point in 2009.
Monique Currie had 19 points and four rebounds to lead the Mystics (9-13), who had won back-to-back road games.
''They're a really big team, especially since they changed the lineup and put Taurasi at the 1 (point),'' Currie said. ''Everybody is six feet and above so that gives a lot of teams trouble but the best you can do is stretch them out and not allow them to use their length.''
Bria Hartley scored 15 points while Ivory Latta added 13 for Washington.
The Mystics trailed by as much as 18 in the second half but was 10-of-14 from the field in the third quarter to stay within 73-60 entering the final period.
''We came out of the fourth quarter a little flat, they made a run, we took a time out, regrouped and came out and we responded - got the dunk, got the crowd into it and after that it was over,'' Griner said.
With more than a month left in the season, the Mercury has a magic number of two to clinch a playoff spot. Phoenix has been in the postseason four of the last five years, reaching the conference finals three times.
Phoenix beat Washington 81-66 on June 10 to sweep the season series. The Mercury have beaten the Mystics four straight times, going back to last year.