Cishek closed it out, pitching 2 1/3 scoreless innings for his fourth save as the Cubs held on for a 6-5 win.
"He's the first guy in line at Dairy Queen," Cubs manager Joe Maddon joked afterward. "That is above and beyond. I was planning on two innings, but then he had to go (get) an extra out."
Cishek retired Rendon with a man on to end the seventh and allowed a single in the eighth before a 1-2-3 ninth. That all came after the reliever spoke with pitching coach Tommy Hottovy, who told Cishek they might need him to throw a couple of innings.
"Jokingly, I told him (I could go) four innings," Cishek said after his longest outing since May 2012. "Good thing that didn't happen."
Kyle Hendricks (4-4) retired the first 11 Washington batters and didn't allow a hit until Kurt Suzuki's single leading off the fifth, but gave up four runs and six hits in 5 2/3 innings. Hendricks had been masterful in May, allowing just one earned run over his past three starts (25 innings).
"Not a good mental approach kind of from the start," he said. "Wasn't attacking as well as I have been the last few games and fastball command kind of suffered from there."
Washington starter Jeremy Hellickson (2-3) allowed three runs and four hits with three walks in three innings.
Rizzo's solo homer in the third gave the Cubs a 3-0 lead, and Chicago went up 4-0 in the fourth on Kyle Schwarber's sacrifice fly. The Cubs scored two more in the sixth to take a 6-1 lead. They had numerous chances to increase their lead in the early innings, but went 1 for 11 with runners in scoring position and left nine men on base -- leaving the door open for Washington.
"The boys are swinging the bat well," Nationals manager Dave Martinez said. "And they faced a guy that came in here pitching really, really well and we were able to knock him out of the game."
Before the game, Maddon announced that the Cubs dropped their protest over the pitching delivery of Washington reliever Sean Doolittle in Saturday night's game.
Rizzo is hitting .315 in 108 career at-bats at Nationals Park, with seven homers and 16 RBI.
Gerardo Parra had three hits. ... Rendon is 10 for 23 over his past six games with two homers, nine RBI and eight runs scored. ... CF Victor Robles made a leaping catch at the wall in right center to take an extra-base hit away from Addison Russell in the ninth.
Cubs: SS Javier Baez left in the sixth inning. He said he jammed his right heel while fielding a grounder by Hellickson in the third and hopes to play Monday.
Nationals: Robles was back in the lineup after being hit on the left wrist with a pitch Friday night and missing Saturday night's game. ... RHP Trevor Rosenthal (viral infection) will pitch an inning for Double-A Harrisburg on Monday and then be re-evaluated. ... 1B Matt Adams (left shoulder strain) has begun swinging a bat and could take batting practice on the field Monday.
Nationals: LHP Patrick Corbin (4-1, 2.91) makes his fourth start of the season against the New York Mets in the opener of a four-gamer series at Citi Field. He is 1-0 with a 2.70 ERA and 24 strikeouts in three starts vs. New York.
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