With the win, the Nationals claimed the three-game series and sent the Rays to their second consecutive loss. The Rays are now 38-31 on the season, 3 1/2 games behind the American League East-leading Yankees.
Peralta entered the game to start the sixth with the score tied at 2, and he retired the first two batters he faced before Jesus Flores doubled to left. Pinch-hitter Adam LaRoche was walked intentionally, bringing Danny Espinosa to the plate.
"I knew they weren't going to pitch to Adam," Espinosa said. "It felt good to have the pressure on me."
Espinosa delivered, yanking a double down the right-field line to send home Flores and LaRoche and give the Nationals a two-run lead.
"I got a good pitch to hit right there, and it just felt good to be able to come through for my team," Espinosa said. "It definitely felt good. To help us get a victory felt really awesome."
Peralta retired the only two batters he faced on Wednesday night and wanted to go back out on Thursday, said manager Joe Maddon.
"[His being used] was based on situational stuff," Maddon said, "because they were going to pinch-hit once the game was tied. At that point you knew the lefties were going to come up there, and I liked him on all those guys. Even Espinosa. ... Tip your cap, that kid's a hard player, man, he's a hard player and he's been better right-handed than left-handed, but he came through for them."
The Rays' best chance to tie the score came in the seventh, when they loaded the bases thanks to three walks. Washington's Ryan Mattheus took over for Mike Gonzalez with two outs in the inning, and Maddon countered by sending the left-handed-hitting Will Rhymes to pinch-hit against the right-hander. Matt Joyce was on the bench with an ailing back, limiting Maddon's options.
Rhymes got ahead in the count, 3-1, but Mattheus came from behind to strike him out swinging to end the threat. Roger Bernadina added an RBI double in the seventh off Wade Davis to increase the lead to three runs.
The Rays got off to an inauspicious beginning offensively. They scored in the second on an RBI single by Desmond Jennings but squandered a chance to get more when Carlos Pena struck out swinging with the bases loaded for the final out of the frame.
Matt Moore started for the Rays and posted two scoreless frames before walking Espinosa and Bryce Harper to start the third. One out later the Nationals pulled off a double steal. Espinosa scored from third on Michael Morse's sacrifice fly, and Ian Desmond singled home Harper with two outs to put the Nationals up, 2-1.
Two mental mistakes by the Rays punctuated that sequence in the bottom of the third. The first came when center fielder B.J. Upton held onto Morse's shallow fly ball too long before throwing home. Second, Desmond's single drew a throw to the plate, but first baseman Pena cut off the throw, got caught napping, and Desmond advanced to second while the ball remained in Pena's glove.
Had Upton made a good throw and nailed Espinosa at the plate, the inning would have ended with no runs being scored. Pena's lapse did not cost the Rays, since the Nationals did not score another run in the inning, but both plays cast a pall over the team's defense, a facet of the game that has been a source of pride for the Rays since they won the division in 2008.
"You know what? That's just bad court awareness by me," Upton said. "I've got to get the ball out of my hands right there. ... It was just a mental lapse. Bottom line."
Maddon said that his team's porous defense of late is "not our DNA."
"We normally play very good defense," he said. "The mental mistakes are the part that's devastating. Physical mistakes happen to everybody on a daily basis. Every Major League game, you're going to see plenty of physical mistakes. You're going to see some mental ones, too, but you don't want to see those. Today we were just chock-full of them."
Upton noted that the team is feeling a lot of frustration about its defense.
"Absolutely. Nobody's out there trying to make an error, but it's almost like they continue to happen. I think as a whole, we're kind of getting over it. ... We've got to do a better job. I feel like we'll get a hold of it."