CINCINNATI — Justin Masterson proved he could pitch in the big leagues, at least in Fenway Park.

The rookie right-hander’s first four starts came at friendly Fenway and he racked up a 3-0 record with a nifty 2.59 earned-run average in those starts.

Last night, Masterson was allowed to take his act on the road, facing the Cincinnati Reds at the Great American Ball Park.

In some ways, though, it was a home game for the 6-foot-6, 250-pounder, who grew up in Beaver Creek, Ohio, about an hour and a half away from the ballpark.

And while he showed flashes of brilliance in front of his family and friends, whiffing a career-high nine in his 6 2/3-inning stint, Masterson and the Boston Red Sox were tumbled by the Reds, 3-1.

Masterson gave up only four hits, but each of the hits went for extra bases, including solo homers by the Reds’ own rookie phenom, Jay Bruce (first inning), and mammoth Adam Dunn (fourth). Back-to-back doubles by Edwin Encarnacion and Joey Votto after Dunn’s laser beam just over the wall in right-center produced the third Cincinnati run.

He was outdueled by Aaron Harang, the Reds’ erstwhile ace who entered the game with a decidedly un-ace-like record of 2-9 and an ERA of 4.31.

Harang was nicked for four hits in seven innings, including a fourth-inning RBI single by Manny Ramirez, who had to leave the game in the seventh because of his sore right hamstring.

“Overall I was happy, but there were three pitches I kind of hung and they made me pay. Those mistakes were hit hard,” said Masterson.

There was some satisfaction in pitching last night in Cincinnati, however.

“I grew up here and it was really cool to come here and pitch in front of family and friends,” said Masterson. “Beaver Creek was well represented. They could share in some of the glory of playing in the major leagues. It was a fun experience, except we didn’t win.”

Masterson’s night couldn’t have gotten off to a more rocky start.

Bruce, the Reds’ leadoff hitter last night, has taken the league by storm since being called up from Triple A on May 27. And while he cooled off a bit, Bruce still was hitting a crisp .391 with three homers and 11 RBI.

In a first-inning rookie-versus-rookie battle, Bruce came out on top, mashing a 1-and-2 pitch over the fence in right field, giving Cincinnati a quick 1-0 lead. It was a slider that had little bite to it, and the location was perfect for a left-handed hitter — low and in.

“You don’t want to start the game giving up a homer, but it wasn’t a bad pitch. He got just enough of it to take it over the fence,” said Masterson.

It was the second time in five starts that Masterson has coughed up a homer to the first batter he has faced. The Rays’ Akinori Iwamura took him deep at Fenway Park on June 3 for a leadoff homer.

But, as he did in that outing, Masterson shrugged off the homer and kept his composure.

He worked around a walk in each of the first and second innings, but limited the Reds to that one hit and one run through three innings.

Along the way he notched seven of his strikeouts.

Harang, meanwhile, was stifling the Sox’ offense.

Boston, though, managed to push the tying run across the plate in the fourth.

Jacoby Ellsbury became the Red Sox’ first base runner when he dropped a leadoff double just fair down the right-field line. He stayed put as Harang got the next two outs, but Ramirez went with an outside pitch and drove it off the base of the right-field fence, tying the game at 1-1.

Ramirez made it to second on Mike Lowell’s single to left, but Votto, the Reds’ first baseman, made a nice play on Sean Casey’s hot smash for the final out of the inning.

That was the extent of the Red Sox’ offense last night against Harang and relievers Jared Burton and Francisco Cordero (13th save).

The Reds’ offense wasn’t busting out at the seams, either. But Cincinnati did enough damage on Masterson’s mistakes to three straight hitters in the fourth — Dunn, Encarnacion and Votto — to pin a loss on the Red Sox.

“Other than a few mistakes, [Masterson] was really good,” said manager Terry Francona. “He got more strikeouts because he’s capable of running the fastball in on right-handed hitters and he has very good off-speed stuff. Unfortunately for us, Harang was a little bit better.”

skrasner@projo.com