PHILADELPHIA -- At the end of a sweltering afternoon, Andrew Knapp kept the Philadelphia Phillies on a red-hot streak.

Knapp hit a game-ending pinch-hit homer just inside the right field foul pole in the 13th inning as the Phillies came from behind to beat the Washington Nationals 4-3 Sunday.

"That was cool," Knapp said. "That was a huge team win -- huge team series."

The win was the Phillies third in this four-game set with Washington. All the wins have come by one-run margins. Over the past two weekends, Philadelphia has gone 5-2 against the two-time defending National League East champions, increasing their edge on the Nationals to three games for second place in the division.

"It shows we're here to stay," Knapp said. "Obviously, there's a lot of games left, and we have to keep playing well. But we're up there with a lot of those teams. This was a huge series win for us, especially coming off the series win in Washington. They wanted to come here and sweep us. For us to take 3 of 4 was cool."

The Nationals are now 6-15 in their past 21 games dating back to June 8. Washington is six games behind Atlanta for the lead in the NL East.

On Sunday, Washington managed only four hits in eight innings against the Philadelphia bullpen, never moving a baserunner past second base in that stretch while striking out seven times.

"We've got to start striking out less, putting the balls in play, driving in runs when we have the ability to drive in runs," Washington manager Davey Martinez said. "But it'll turn. Our offense is good. I know it's good. And I truly believe that it'll turn around."

Knapp ended the game when he turned on a Justin Miller fastball and lofted it into the bleachers in right field. Miller (5-1) gave up the run in 1 1/3 innings of work.

Nick Pivetta (5-7) got the win pitching a scoreless inning. Pivetta- normally a starter- was roughed up for seven runs in 1 2/3 innings of work on Friday night. He entered the game as the final pitcher that Philadelphia had available.

"He wanted the opportunity bad," Phillies manager Gabe Kapler said. "I asked him and pushed him to make sure that this is something that he was going to fight for and he did."

Washington jumped out early, scoring an unearned run in the fourth on an Anthony Rendon sacrifice fly. In the fifth, the lead expanded to 3-0 when Adam Eaton singled to right to score Daniel Murphy and Mark Reynolds.

However, Gio Gonzalez labored in the bottom of the fifth after cruising through the first four innings with only one hit and one walk allowed. He walked three guys in the fifth inning, including Cesar Hernandez with the bases loaded, that put Philadelphia on the board. Rhys Hoskins' sacrifice fly to right made it 3-2 then Odubel Herrera singled to score Carlos Santana to tie the game.

GAME NOTES

Knapp became the first Phillie to hit a walk-off, pinch-hit homer since Cody Asche did it back on Sept. 12, 2015 against the Chicago Cubs. Ryan Howard hit the last walk-off homer for Philadelphia back on April 29, 2016. . The Phillies bullpen posted a 0.60 ERA with 13 strikeouts in 15 innings pitched on Saturday and Sunday. .. Nationals starter Gio Gonzalez has never allowed more than three earned runs in 14 career starts at Citizens Bank Park. He has tossed five or more innings in every start.

TRAINERS ROOM

Philadelphia placed right-handed starting pitcher Vince Velasquez on the 10-day disabled list on Sunday with a right forearm contusion.

Velasquez was hit in the second inning just below the elbow on a liner back up the middle by Washington's Adam Eaton. Velasquez would eventually record the out by picking the ball up and throwing it lefthanded to first base to get Eaton to end the inning, saving a run from scoring during Philadelphia's 3-2 victory.

This is the fourth trip to the disabled list for Velasquez in three seasons since Philadelphia acquired him.

In addition to Velasquez's trip to the disabled list, the Phillies optioned lefthanded reliever Zac Curtis to Triple A Lehigh Valley. They recalled right-handed reliever Jake Thompson and reinstated reliever Pat Neshek from the 10-day disabled list. Neshek, who was signed by the Phillies during the off-season, has been on the disabled list since Opening Day with a forearm injury.

UP NEXT

Nationals: Washington returns home on Monday night to begin a three-game interleague series against the Boston Red Sox. RHP Max Scherzer (10-4, 2.04 ERA) will face former Detroit teammate Rick Porcello in a battle of former Cy Young Award winners.

Phillies: Philadelphia has Monday off, but will begin a brief two-game interleague series at home against the Baltimore Orioles on Tuesday night. Zach Eflin (6-2, 3.02 ERA) will get the start against Baltimore's Alex Cobb.

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