Lucas Giolito pitched five innings and allowed four runs in a 5-3 loss to the Rockies. (Toni L. Sandys/The Washington Post)

It was the third inning of the Washington Nationals5-3 loss Sunday afternoon, and Lucas Giolito seemed to have survived the turbulence. The Nationals’ top prospect had settled in after a rocky first inning, one pitch away from dismissing the most dangerous section of the Colorado Rockies’ lineup. It would have been a modest triumph but one nonetheless for a prized hurler who had never thrown a pitch in the fifth inning in the majors before.

Giolito, who was called up from Class AAA Syracuse earlier Sunday, was complementing his fastball with change-ups and curve­balls successfully, and batters were not fouling off his best pitches. The evidence was in his swing-and-miss total: He compiled five in the second and third innings after generating one in his previous outing at this level.

Then, ahead in the count 0-2 on the slugging Nolan Arenado, the right-hander accepted a fastball sign from catcher Wilson Ramos and delivered a 94-mph offering. He missed his intended target, up and in, and Arenado was not fooled. He launched a laser into the bed of red flowers just over the left field wall at Nationals Park for a two-run home run and the second of his four hits. The next pitch was another fastball, Giolito’s fifth straight and a tick slower at 93 mph, and David Dahl smacked it into the Rockies’ bullpen beyond the left field wall.

Two straight fastballs, two missed locations and three instant runs. Suddenly, the 22-year-old Giolito’s promising start confronted a roadblock on a day the Nationals needed him to go deep into the game because their bullpen remained drained in day 17 of Washington’s stretch of 20 games in 20 days. Giolito regrouped over the next two innings, but the rapid damage was a cruel reminder that the margin of error in the major leagues is thin.

Bryce Harper celebrates after hitting a solo home run in the ninth inning. It proved to be too little too late. (Toni L. Sandys/The Washington Post)

“I just need to get better about getting ahead of guys and finishing guys off with the pitch I want to throw and make sure I hit my spots,” Giolito said.

The 6-foot-6 Giolito allowed four runs on six hits across a career-high five innings. He struck out two and walked two. He took the ball Sunday having thrown a fastball 75.1 percent of the time in his first three career outings, according to FanGraphs. The only qualified pitcher with a higher percentage in the major leagues is 43-year-old craftsman Bartolo Colon, who throws fastballs 89 percent of the time, and his success bewilders all. The one-sided ratio was perhaps a factor in Giolito getting hitters to swing and miss just 6 percent of the time as a major leaguer entering Sunday. Giolito’s whiff count Sunday: six out of 100 pitches .

“They didn’t miss his fastball,” Nationals Manager Dusty Baker said. “That was the key.”

The Nationals mustered just two runs over seven innings against Rockies right-hander Chad Bettis, and all three of their runs came via solo homers as they lost for the second consecutive day to a team that almost certainly won’t play in the postseason.

But defeats haven’t hurt Washington (75-55) in the standings because the Miami Marlins also have lost two straight. As a result, the Nationals’ lead in the National League East remains eight games as they begin a stretch of 22 consecutive games against division opponents.

Trea Turner immediately gave Giolito the run he surrendered in the first inning back in the bottom half of the frame with his first career leadoff home run. The homer was Turner’s fifth and hiked his August run total to 27, setting a franchise record for the most in a month for a rookie. Turner later singled in the fifth to post his sixth straight multihit game. The mark is tied for the longest in baseball this season.

Nationals first baseman Ryan Zimmerman goes up to catch a throw from pitcher Lucas Giolito to retire the Rockies’ Chad Bettis in the fourth inning. (Toni L. Sandys/The Washington Post)

But Washington didn’t score again until Ramos smashed a first-pitch fastball from Bettis 450 feet to center field to lead off the seventh inning for his 20th home run. Bryce Harper added his own solo home run in the ninth — his 23rd of the season and third in 15 games since coming back from a neck injury. Besides the three home runs, the Nationals tallied three hits and didn’t have a runner reach third base.

“We hit some balls pretty good, but they hit some balls really good,” Baker said.

Giolito was officially called up Sunday morning, and infielder Wilmer Difo was optioned to Class AA Harrisburg to create a spot on the 25-man roster. The transaction left the Nationals with a short bench and without a backup infielder besides left-handed first baseman Clint Robinson.

Left-hander Oliver Perez replaced Giolito in the sixth and surrendered hits to the first two batters he faced before wiggling out of the predicament by inducing three straight groundballs to keep the score 4-1.

Ramos cut the deficit with his home run in the seventh, but the Rockies got the run back in the most peculiar of ways. With Daniel Descalso at second base and two outs, right-hander Koda Glover unleashed a 96-mph fastball to pinch-hitter Ryan Raburn to begin the at-bat. The pitch sailed past Ramos, who was expecting a different pitch, and hit home plate umpire Mike Muchlinski squarely on his left shoulder. Muchlinski crumpled to the ground in pain. Ramos initially checked on him instead of retrieving the ball. Recognizing the play wasn’t dead, Descalso raced around third and scored from second without a throw. Descalso’s decision drew boos from the crowd, but the run counted.

“Honestly, I don’t know how that run’s able to score,” Glover said. “At most, I thought he’d be told to go to third.”

Giolito was charged with the loss, his first major league decision. Though the performance was an improvement from his previous starts in a Nationals uniform, Giolito said he didn’t think of it that way because he had pitched better recently. He commanded all his pitches and put hitters away in his last three minor league outings. He was living up to expectations, and he wanted to replicate that for the Nationals on Sunday. The Nationals are still waiting.