Matt Wieters hit a tiebreaking grand slam as the Nationals rallied for five runs in eighth inning and went on to defeat the Cubs 9-4 on Sunday.
"We felt it was just a matter of time before our lineup can put together an extended inning on them," said Wieters, who had five RBIs. "We were able to do that later in the game today."
The switch-hitting Wieters stepped over to the left-side batter's box against the right-handed Edwards and followed with his eighth homer of the season and third career grand slam on the first pitch.
"It was a first-pitch breaking ball," Wieters said. "But out of his hand it was something I recognized I felt I could get into the air and it just happened to carry out."
Brian Goodwin added a solo shot and had two RBIs and Rendon drove in two runs as the National League East-leading Nationals rebounded to take two of three games in Chicago.
"We always try to win the seventh, eighth and ninth," Nationals manager Dusty Baker said. "It doesn't always happen and we did it against a pretty good bullpen."
The Nationals might have gotten an extra boost because they could meet the Cubs in the NL playoffs.
"Who knows, that could maybe come up big," Baker said.
"It doesn't count for the team and we still lost the game," Contreras said. "I don't care if I hit two homers."
Javier Baez also drove in a run for the Cubs, who are 15-7 since the All-Star break and remained a half-game ahead of second-place Milwaukee in the NL Central.
Chicago's Jason Heyward had three hits, including the 1,000th of his career.
Contreras has homered five times in his last four games and also went deep twice on Thursday against Arizona. The Cubs' energetic second-year catcher has homered 10 times since the All-Star break and belted 16 of his 21 home runs this season since June 19.
Brandon Kintzler (3-2), the second of four Washington relievers, pitched a scoreless seventh to pick up his first win since coming to the Nationals in a trade from Minnesota.
Edwards (3-3) allowed four runs in two-thirds of an inning in taking the loss. The slim righty has given up seven runs in 1 2/3 innings in his last three outings.
"This game is all built around confidence," Lester said. "He maybe doubting himself a little bit right now. He'll get back to being himself. As a reliever, it get magnified."
Lester pitched 6 2/3 innings and allowed three runs. The veteran left-hander struck out seven and has fanned 36 in 32 2/3 innings since the All-Star break.
Nationals right-hander Erick Fedde allowed four runs and eight hits in 5 1/3 innings in his second big-league start. The 24-year-old was recalled from Triple-A Syracuse to fill in for ace Max Scherzer, whose start was pushed back to Monday because of discomfort in his neck.
"I like the way Fedde gave us a chance to come back," Baker said.
Fedde retired the first five hitters his faced, striking out three. But Ian Happ, Heyward and Baez singled with two outs in the second to put Chicago ahead 1-0. Fedde fanned Lester to end the threat.
Washington's Wilmer Difo lined a triple just beyond the reach of center fielder Jon Jay to lead off the third. He came home to tie it at 1 when Schwarber couldn't hang on to a sinking liner in left field.
Contreras crushed his first homer to the top of the left field bleachers on 2-1 pitch to lead off the fourth to put Chicago back ahead 2-1.
Conteras went deep again off Fedde -- this time midway up the left-center bleachers -- to lead off the sixth. Schwarber followed with his 18th homer into the first row in right center to make it 4-1.
The Nationals trimmed it to 4-3 in the seventh on a doubles by Daniel Murphy and Rendon and a sacrifice fly by Difo.
After jumping ahead 8-4 in the eighth, Goodwin added a solo shot in the ninth.
NOTES: Nationals RHP Erick Fedde, 24, was drafted 18th overall in 2014 and is rated as the No. 2 prospect in Washington's system by Baseball America. The 6-foot-4 right-hander made his major league debut on July 30 against Colorado and allowed seven runs in four innings in a loss. ... Chicago 3B Kris Bryant was limping slightly after running out a groundout in the first inning but stayed in the game. ... The Cubs recalled INF Tommy La Stella from Triple-A Iowa before Sunday's game. LHP Rob Zastryzny was optioned to the farm club. The 28-year-old La Stella joined Chicago for the fourth time this season. He is hitting .298 with six RBIs in 37 games with the Cubs. ... Each Cubs player had a leather motorcycle vest bearing his nickname hanging in his locker before the game as Chicago begins another themed road trip at San Francisco on Monday night. ... Emmy Award-winning actor William H. Macy threw out the first pitch and led the crowd in "Take Me Out to the Ballgame" during the seventh-inning stretch.