Philadelphia's Sean O'Sullivan has done it three times this season, and the Phillies have lost each time, including a 2-1 setback Friday night.
"Yeah, I did look to see who I was facing again and I went oh, ok," O'Sullivan said. "At least I know what I'm up against when I got out there."
Harper hit his 16th home run, 11th in 14 games overall and third against O'Sullivan in three games this year.
Though the underdog in the pitching matchup, O'Sullivan (1-3) held his ground, allowing two runs and five hits in six innings.
Allowing a homer to a slugger like Harper is one thing. Giving up a single to a fellow pitcher who then scores the go-ahead run is another.
Scherzer helped his own cause with his bat and legs. With the score tied at 1-1, he led off the sixth inning with a clean single to right. One out later, the pitcher raced home from first base on Ian Desmond's double and the Nationals went on to their sixth straight win.
"For him to be the go-ahead run kind of stings," O'Sullivan said.
The Phillies have lost three of four after winning six in a row.
Scherzer (5-3) allowed four hits over eight innings with six strikeouts and one walk. He retired 19 of 22 after Odubel Herrera's RBI double in the second — including the final eight batters. Over his last four starts, Scherzer is 4-0 with 33 strikeouts.
Scherzer wasn't totally untouchable in his third start this season against Philadelphia. Three of the four hits he allowed were doubles. Denard Span chased down multiple long fly balls in center field.
"I thought we hit some balls hard off of (Scherzer)," Phillies manager Ryne Sandberg said. "We hit some balls off the wall. I thought we attacked him pretty well, but yeah he's top notch."
O'Sullivan went 0-1 with a 4.91 ERA in two earlier starts against Washington this season, both coming in April. Harper homered in each game. The right fielder did it for the third time in his first at-bat Friday.
The Phillies opened the scoring with two wall-smacking doubles in the second inning for a 1-0 lead. Harper immediately countered leading off the bottom of the inning, poking a 1-2 pitch into the left field stands.
Overall, O'Sullivan kept his side in the game.
"He threw strikes, stayed ahead of the hitters," Sandberg said. "Solid game, pitcher's duel, pretty solid game all around even having a chance down to the end."
The Phillies scored four or more runs in seven of their previous eight games.
Utley had two hits in the second stop of a three-city, nine-game road trip.
Philadelphia fell to 3-5 against Washington this season — including the three losses in games started by Scherzer.
"Scherzer with his 'A' game is tough to beat," O'Sullivan said.
He would know better than most.
Phillies: OF Grady Sizemore went 0 for 4 in his return to the starting lineup for the first time in three games.
Nationals reliever Aaron Barrett and Phillies pitcher Aaron Harang continued a pre-game standoff all the way through Scherzer's first pitch. Wearing a batting helmet, Barrett stood in front of the Nationals dugout for several minutes. Harang, who donned a catcher's mask and chest protector, stood some feet away from the Phillies' dugout. Eventually Harang gave in, leading to roars from the stands and Barrett's teammates.
Phillies: LHP Cole Hamels (4-3, 3.24 ERA) is 3-0 with a 1.69 ERA over his last three starts. He is 0-1 with a 4.15 ERA in two starts against Washington this season.