Cody Stanley, called up earlier in the day from Triple-A Memphis, started the rally with a pinch-hit double. Tommy Pham walked, setting up Moss' fifth career walk-off homer and his second walk-off hit as a Cardinal.
"That obviously felt good, but it felt even better because my first two at-bats were runners in scoring position, bases loaded, and my first at-bat I got punched out looking, and that's a terrible feeling," Moss said. "So to come through right there feels a lot better because of that, to kind of redeem yourself."
Both starters went just 2 2/3 innings. Cardinals lefty Marco Gonzales gave up four runs -- all coming in the third, fueled by Anthony Rendon's two-run double and Ryan Zimmerman's solo homer -- on seven hits, while Nationals righty Joe Ross gave up three runs on one hit, but he issued six walks.
Mitch Harris (2-1), also called up earlier in the day from Memphis, got the win with two scoreless innings of relief. Casey Janssen (1-4) took the loss for the Nationals.
"Mitch Harris came in, and we needed help tonight and we needed him to go multiple [innings] for us in a high-leverage situation, and that was a good sign," Cardinals manager Mike Matheny said. "He was letting the fastball go, the breaking ball looked sharp. It was just a good day."
MOMENTS THAT MATTERED Sweet relief: Pitching for the first time since Aug. 21 and only for the second time in 16 days, Cardinals right-hander Carlos Villanueva turned in 3 1/3 innings of scoreless relief to keep the Cardinals in the game. Villanueva gave up one hit and struck out six of the 11 batters he faced.
"I knew I was going to try to go multiple innings there, just keep the game where it was," Villanueva said. "The way we've been playing, the way we've been scoring runs in bunches, you kind of feel like the boys are going to come back." More >
Ross loses grip, but Fister to the rescue: After Ross struggled and made his early exit, Fister pitched 2 1/3 scoreless innings of relief. He struck out Moss to end the third inning with the bases loaded after he hit Mark Reynolds with a pitch to force in a run, charged to Ross, who had problems gripping the ball. It didn't help that he didn't use the rosin bag.
"The first two innings were all right. I was kind of sweaty wiping my hand off. The pants were kind of sweaty. I just had a tough time finding the zone," Ross said. "I will probably use the rosin [bag]. That's not something I've done in the past. That's something you have to do to keep your hand dry."
Taking what they're given: The Cardinals scored three runs in the third and two more in the eighth, and both rallies occurred with just one hit each. The Cards scored two runs in the third on a bases-loaded walk and a hit batter. In the eighth, they capitalized on a throwing error and tied the game on a double play.
"We put some pressure on them," Moss said. "We had some really good at-bats late. We had good at-bats all night. We weren't getting a lot of hits, but we were working their pitchers, making them throw pitches and putting pressure on them."
Costly error: With runners on first and second and no outs in the eighth, Cards second baseman Greg Garcia bunted toward reliever Drew Storen, who tried to get the forceout at third. But the ball went past Yunel Escobar for an error, allowing Pham to score and make it 5-4. The throw from Storen wasn't offline, and it looked like Escobar may have had a chance to catch the ball.
"It's a judgement call," Storen said. "You see how it comes off the bat. Lobby [Jose Lobaton] said three. So you just turn and go."
The bases were loaded with no outs when Stephen Piscotty then hit into a double play, scoring Moss to tie the game at 5. More >
"We had a two-run lead in the eighth. We didn't get it done. We used everybody to get there. ... We had the score tied, and at that point, we have to go to Casey. -- Nationals manager Matt Williams, who used five relievers before going to Janssen
Reynolds left the game in the bottom of the sixth inning after his hit by pitch in the third, and he was diagnosed with a right wrist contusion. X-rays were negative, and he is considered day to day.
UPON FURTHER REVIEW
Matheny successfully challenged a safe call at first base in the top of the ninth inning. With a runner on first, Zimmerman grounded to first and was called safe by umpire John Tumpane, but after a 52-second review, it was ruled that Moss' flip to Harris beat Zimmerman to the bag. The ruling loomed large as the Nationals ended up stranding runners on first and third without scoring.
WHAT'S NEXT Nationals: Born in St. Louis, Nationals right-hander Max Scherzer will pitch at Busch Stadium for the second time in his career in Wednesday's finale vs. the Cardinals at 8:15 p.m. ET. His first start at Busch Stadium occurred on Sept. 24, 2008, when he was a member of the D-backs. He pitched five innings, allowing four runs (two earned) in a 4-2 loss. Scherzer is looking to break a personal three-game losing streak, having not won a game since July 30 against the Marlins.
Cardinals: Righty Michael Wacha (15-4, 2.69 ERA) takes the mound in the finale of the three-game series Wednesday at 7:15 p.m. CT. Wacha is 2-1 with a 0.79 ERA in three career starts against the Nationals. On Sept. 24, 2013, he carried a no-hitter into the ninth against the Nats at Busch Stadium before it was broken up by a two-out infield single.
Joe Harris is a contributor to MLB.com. Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the Time. He also can be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.