PHILADELPHIA -- This 10-day span could end up serving as a season-defining stretch for the 2019 Washington Nationals. Friday marked the beginning of a grueling 10-game road trip that starts in Philadelphia and goes through Milwaukee before finishing up in Los Angeles -- a chance for Washington to measure itself

PHILADELPHIA -- This 10-day span could end up serving as a season-defining stretch for the 2019 Washington Nationals. Friday marked the beginning of a grueling 10-game road trip that starts in Philadelphia and goes through Milwaukee before finishing up in Los Angeles -- a chance for Washington to measure itself against some of the top competition in the National League.

The margin for error is even smaller in games like these, amplifying crucial pitching decisions such as the one that led to Friday’s 4-2 loss to the Phillies at Citizens Bank Park, when left-hander Dan Jennings surrendered a three-run homer to Rhys Hoskins in the sixth inning that would serve as the decisive blow.

• Box score

The inning began with Jeremy Hellicksonon the mound, on a roll and in the midst of one of his best games in years. He had collected nine strikeouts -- his most since 2017 and one shy of his career high -- but he was scheduled to face the top of the Phillies' lineup for the third time. So after Jean Segura 's infield single down the third-base line, Martinez went to his bullpen against the heart of the Philadelphia order, much to the displeasure of Hellickson.

“There's gonna be games where that's probably the right move,” Hellickson said. “But the way I was going today, I think that was my inning. I thought I should’ve gotten a chance to get out of there. I gave up maybe one hard-hit ball since the first inning.”

Indeed, Hellickson had limited the Phillies to just four hits in 5 1/3 innings, but the only hard-hit ball he had surrendered after the first inning was a 110.7 mph double off the bat of Bryce Harper , who was due to hit next. The Nationals have routinely limited Hellickson’s exposure to opposing lineups a third time through the order where he surrenders an .846 career OPS compared to a .670 OPS the first time through and .729 the second.

So manager Dave Martinez went to Jennings to get the lefty matchup he wanted with one out in the sixth. But Jennings, whom the Nats signed to a Minor League deal in mid-April, walked Harper on five pitches, setting up a showdown with Hoskins -- another matchup Martinez said the Nationals had identified before the game.

Martinez was comfortable allowing his lefty to face the right-handed Hoskins because of Hoskins’ reverse splits -- .194/.376/.392 in his career against lefties -- and because Hoskins had struggled particularly against sliders from lefties, going 0-for-17 with nine strikeouts in his career, according to Statcast.

“With the slider, he had zero slug on left-handed sliders,” Martinez said. “And Jennings has got a really good slider. That’s the matchup. We liked that matchup.”

It didn’t work. Jennings threw a 1-1 slider at the bottom of the zone, right in the wheelhouse of the Phillies' slugger, who turned on the pitch and deposited it into the left-field stands.

"I know I have some reverse splits, which is frustrating to me just because I usually hit lefties well throughout the rest of my career,” Hoskins said. “I was just excited to finally do some damage against a lefty."

The thought process behind the decision was sound, but the players did not execute, leaving the Nationals walking away with a loss for the 10th time in 14 games.

“You get him out, you don’t really second-guess anything,” Jennings said. “Maybe it’s a pop fly right there, and I don’t second-guess the pitch at all, but naturally [if] it doesn’t go your way, you start second-guessing pitches a little bit and that’s really what I’m there to do.”

Jamal Collier has covered the Nationals for MLB.com since 2016. Follow him on Twitter at @jamalcollier.