"It's absolutely epic. That's the best word I can put out there," Harper said.
Five of Washington's past six victories have been by a single run, with four coming in its last at-bat.
"It's been incredible," Harper said. "This team has a lot of fight, a lot of heart. We never die."
Harper opened the ninth with a single off Evan Marshall (4-3) and went to third base on Kevin Frandsen's one-out single. Out of the starting lineup for the first time in 63 games, Rendon singled past third baseman Cliff Pennington to win it.
"It was different to say the least, but it was awesome," Rendon said of the pinch hit.
Neither epic nor awesome describes what manager Matt Williams feels watching the slew of walk-off wins.
"It's gratifying for (the players), it's exciting for them," Williams said. "It's not a whole of fun in the dugout for the staff, but that's OK."
The Nationals had last won nine in a row when they matched a franchise record by winning 10 straight in June 2005.
The National League East leaders also extended their home winning streak to eight and opened a seven-game lead over Atlanta.
Rafael Soriano (3-1) pitched the ninth for the win.
Arizona has lost five straight and is 5-13 in August.
"In the end, they out-executed us in every phase of the game," Diamondbacks manager Kirk Gibson said. "They pitched good, and we didn't pitch good enough. They out-fielded us. They outhit us."
Two of Washington's wins in this series have come in its last at-bat after Arizona tied the game late.
"Of course it's disappointing," Gibson said. "They've got it rolling and they overtook us again."
Tanner Roark threw seven scoreless innings for the Nationals and left with a 2-0 lead. He gave up five hits. Roark is 5-1 with a 2.09 ERA over his past eight starts.
Drew Storen replaced Clippard with one on in the eighth, eventually stranding the runner at third base.
Arizona starter Trevor Cahill allowed two runs and six hits over 6 1/3 innings.
Harper had three of Washington's nine hits.
The game started 12 minutes late due to rain.
Nationals: Williams sounded hopeful that 3B/OF Ryan Zimmerman could return "sometime in September" from a hamstring injury. Zimmerman was placed on the disabled list July 23.
Diamondbacks: In his fourth rehab appearance, Daniel Hudson tossed one scoreless inning with two strikeouts Tuesday night in the Arizona Rookie League. The right-hander hasn't pitched at the major league level since 2012 after sustaining an elbow injury that resulted in Tommy John surgery twice.
NOT JUST A PITCHER
Leading 1-0 in the sixth, Roark deftly fielded Miguel Montero's grounder to start an inning-ending 1-6-3 double play after the Diamondbacks put runners on the corners with one out.