WASHINGTON

Just stepping into the batter’s box to face good friend and former teammate Max Scherzer was a special occasion for Rick Porcello.

A career American League pitcher with 35 plate appearances on his resume going up against a three-time Cy Young Award-winner had mismatch written all over it. Instead, Porcello hit a three-run double off Scherzer that set the stage for the Boston Red Sox beating the Washington Nationals 4-3 in the opener of their three-game interleague series Monday night.

“I got lucky,” Porcello said. “He got to the top of his windup and I told myself, ‘Start swinging.’ Then I hit it.”

Porcello’s first major league extra-base hit came in the second inning on an 0-2 pitch after the Nationals intentionally walked Jackie Bradley Jr. to pitch to him. His .156 batting average and limited hitting experience made it appear to be the right calculation, but Porcello drove Scherzer’s 96 mph fastball over the head of Juan Soto in left center for his first RBIs since 2009.

“You don’t hit for an entire year,” said Porcello, who came in with five hits and two RBIs in his career. “You don’t even know what it’s going to look like once you step in there. I was thinking swing the bat and be competitive up there. You just never know what’s going to happen because you don’t do it very often.”

Scherzer and Porcello were teammates for five seasons with the Detroit Tigers and are still close. Because of that, Scherzer wasn’t surprised by Porcello’s hitting stroke.

“I know he can hit,” Scherzer said. “Give him a couple sliders to keep him off-balance and then was trying to get a fastball up and away and it ran back middle-in. Anybody can hit middle-in. He can do that. I’ve seen him do it.”

On the mound, Porcello (10- 3) struck out five and limited the damage to solo home runs by Anthony Rendon and Daniel Murphy. Scherzer (10- 5) struck out nine and kept the Red Sox off the board except for Porcello’s bases-clearing double and became the 11th pitcher to reach 1,000 strikeouts with two different teams.

When Scherzer exited the game after 108 pitches, Mookie Betts provided the Red Sox with their insurance run with a solo shot off Brandon Kintzler in the seventh, his 21st home run of the season. Bryce Harper hit his 21st in the eighth, but Boston’s bullpen held on to hand the Nationals their 16th loss in 24 one-run games.

“Go out there and try to win ballgames and they all feel the same to me,” Harper said as Washington fell to 42-41, seven games back of NL East-leading Atlanta. “I mean we lose the game, you’re a loser. Win the game, you’re a winner.”

Craig Kimbrel got into some trouble but picked up four outs for his 25th save in 27 chances.

FIRST-BASE HARPER? — Harper is doing just fine in center field but on Monday fielded some grounders at first base. It’s just experimental for now but could help the Nationals out down the line.

“I don’t ever know if he’ll ever start a game there, but I believe that if we’re in a pinch and we’ve got to make some kind of decisions in-game, he probably could play there,” manager Davey Martinez said.

TRAINER’S ROOM — Red Sox: RHP Steven Wright (left knee inflammation) received a PRP (platelet-rich plasma) injection, a day after throwing a short bullpen session. Cora said Wright could return before the All-Star break. . 2B Dustin Pedroia (left knee inflammation) will remain in New York all week. . Cora said RHP Tyler Thornburg (shoulder surgery) will most likely be activated from the disabled list during this series.

UP NEXT— Boston LHP Brian Johnson (1-2, 4.28 ERA), who is 1-0 with a 3.18 ERA in three career interleague appearances, faces Washington for the first time. RHP Tanner Roark (3-9, 4.10 ERA) pitches for the Nationals in his first career start against the Red Sox.