The Nationals’ Denard Span, left, dives safely back to first base as Cincinnati’s Joey Votto, right, waits for a throw from pitcher Johnny Cueto. (Tom Uhlman/Associated Press)

Stephen Strasburg ambled from the on-deck circle to home plate in the fifth inning Sunday afternoon, a walk the Washington Nationals had rarely allowed him. Strasburg had thrown 92 pitches — 12 more than Manager Davey Johnson permitted six days earlier — and the go-ahead run stood on second base in a tie game. But Johnson has long spoken about looking to Strasburg, 24, as the leader of his staff. Now came time to act on that vow.

“I figured he could give me another inning, or maybe even two,” Johnson said. “He’s my horse.”

Johnson’s decision to unshackle Strasburg led to the Nationals’ undoing in the sixth and, ultimately, a 6-3 loss at Great American Ball Park that allowed the Cincinnati Reds to claim an early April showdown. The Reds, with help from a crucial miscue by second baseman Danny Espinosa, ambushed Strasburg for three runs in the fifth as he recorded only one out, capping one of the rockiest starts, by the numbers, of Strasburg’s young career.

Against the Reds’ meat grinder of a lineup, Strasburg yielded six earned runs — three in the first inning and three in the fifth — while allowing a career-high 13 base runners: nine hits and four walks. He fired 114 pitches, the third most of his career. The Nationals’ lone offense against Reds ace Johnny Cueto and a flame-throwing bullpen came on catcher Kurt Suzuki’s three-run homer in the second inning.

The Reds, winners of 97 games last year, claimed the early edge between two teams who figure to circle one another all year. They will meet again in Washington at the end of April, and they may well play in October. This weekend, the Reds took the series and outscored the Nationals, 27-10.

“We didn’t play our best by no means, and we were right there with them,” shortstop Ian Desmond said. “I think when we go back to D.C. and play them again, I think it’s going to be a good series. They obviously have a good team. We didn’t put our best foot forward.”

The finale turned in the fifth, when Johnson stuck with Strasburg. On opening day, Johnson removed Strasburg from a game the Nationals led, 2-0, in the seventh inning after he had thrown only 80 pitches. Sunday, Johnson believed, was different. The Nationals had used four relievers in an 11-inning victory Saturday, and so Johnson thought he needed at least six innings from his starter. Johnson also thought the adrenaline from an opening day start sapped more energy.

“Opening day, that was like throwing 95 [pitches],” Johnson said. “But today I thought he was free and easy, nice and loose. He’s a pretty good hitter, too.”

Cueto struck out Strasburg and Espinosa, on second after a double, trotted to the dugout. Strasburg grabbed his glove and took the mound. Last year, he began an inning after throwing 90 pitches in only three starts.

“That’s what I want to do,” Strasburg said. “I want to go out there and go deep in the ballgame. . . . Some days, you’re cruising and you’re not even at 100 pitches through seven. There are some games when you’re right around 100 through five. You still want to go out there and try to get through at least two more innings.”

Sunday, Strasburg didn’t make it through one. Rookie Derrick Robinson led off with a pinch-hit single to left, the first hit of his major league career. Shin-Soo Choo followed with a liner up the middle for a single, and Robinson, who once played defensive back at Florida, blazed all the way to third.

Strasburg’s pitch count sat at 98. Johnson drew the middle infielders to “halfway” depth — closer to home than usual, not quite on the grass. At second base, Espinosa predetermined that if a hard-hit ball came to him, he would throw home to try to prevent the go-ahead run. Johnson, though, wanted to concede the run and turn the double play.

“When you go halfway, the rule of thumb is: hard-hit ball, turn two,” Johnson said.

Xavier Paul rolled Strasburg’s 101st pitch, a 2-0 fastball, to the right side. Espinosa fired home.

“It was just my misunderstanding of why we were playing halfway,” Espinosa said afterward. “I thought the reason we were playing halfway was to try to cut that run.”

With Robinson sprinting on contact, Espinosa’s throw had to be perfect. It carried catcher Suzuki’s mitt to the right side of home plate, allowing Robinson to slide home with the go-ahead run.

Teammates covered for Espinosa. Suzuki insisted he could have been better positioned to make a tag. Desmond believed he could have helped Espinosa make the correct play.

“I probably should have been saying, ‘Two! Two! Two!,’ getting his attention,” Desmond said. “As infielders, when we’re out there, we see our pitcher battling, you want to try to stop all runs at all costs.

“They got a little bit of a slow track there on the grass. It’s cut a little longer than ours. I think the ball took a little bit longer to get to Espinosa. At [Nationals Park], he gets that guy out at home 10 times out of 10. Probably a little bit my fault, not letting him know we should turn two.”

Strasburg kept firing. He retired Joey Votto on seven pitches, raising his count to 108. Johnson still stuck with him. Brandon Phillips fought through a six-pitch at-bat and ripped a 2-2 change-up through the left side of the infield. Choo scored, the Reds took a 5-3 lead and, after throwing 114 pitches, Strasburg jogged off the mound.

“There was something going on,” Strasburg said. “When I’m out of the stretch, they seemed to take better swings. I just got to look at the video and see what I’m doing out there.”

Was he tipping pitches, perhaps?

“I’m just going to look at some video and see what happens.”

From the start, the Reds made life difficult for Strasburg. In the first inning, Paul dribbled a hit off Strasburg’s glove, Votto drew a five-pitch walk and Ryan Zimmerman dived to knock down Phillips’s blistering grounder but couldn’t corral it. Suddenly, without a ball leaving the infield, the Reds had loaded the bases. Jay Bruce poked a 2-2 curveball to the gap in left-center field, a two-run double. Todd Frazier’s RBI groundout made it 3-0 after the first.

“I thought he threw the ball good,” Suzuki said. “He gets a little frustrated, you know, giving up a lot of soft contact. Spins, bounces, some hits, broken bat stuff didn’t real go our way today. All he can do is make quality pitches and create soft contact early in the count. And that’s what he did.”

Strasburg may have another chance to navigate the Reds’ ferocious lineup, and Johnson assuredly will continue to stretch his limits. He could not lift the Nationals on Sunday. He is still their horse.