Shawn Horcoff and Tyler Seguin also had man-advantage goals, and Colton Sceviour also scored as Dallas completed its season-high, five-game road trip with three straight victories. The Stars won four of five overall.
The three power-play goals are the second most the Stars have scored in a game this season. They netted four in a 5-2 victory over Winnipeg on Jan. 31.
Kari Lehtonen made 18 saves and has won six consecutive games versus the Capitals. The Stars have beaten Washington seven straight times.
Curtis Glencross and Evgeny Kuznetsov scored for the Capitals. Braden Holtby stopped 18 of 22 shots before being replaced after Fiddler made it 4-1 in the second period. Justin Peters stopped all nine shots he faced in relief.
Dallas started quickly, recording the first six shots and holding Washington to one for over nine minutes.
The Stars finished with a 31-20 edge in shots.
Dallas took a 1-0 lead on Horcoff's power-play goal 3:25 into the game. His shot deflected off Glencross and went over Holtby's right shoulder.
Glencross tied it for Washington before the Stars took the lead back when Sceviour scored for the first time in 17 games and gave Dallas a 2-1 lead after one period.
The Stars added two more power-play goals in the second period. Seguin got the first before Fiddler added the next for a 4-1 lead with 5:55 left.
Washington coach Barry Trotz then replaced Holtby with Peters, who hadn't played since Feb. 15.
Kuznetsov added a late goal for Washington. Alex Ovechkin earned his 418th NHL assist, which tied him with teammate Nicklas Backstrom and Michal Pivonka for first place in team history.
NOTES: Capitals D Brooks Orpik missed his second straight game due to an undisclosed injury. He played in Washington's first 67 games. ... The Capitals recalled C Andre Burakovsky from Hershey of the AHL and reassigned D Cameron Schilling there. Burakovsky was in the lineup and had one assist. ... The Stars have won four straight in Washington. . Dallas RW Ales Hemsky tied a season high with two assists.