Jim Riggleman argues with home plate umpire Todd Tichenor during the first inning. Riggleman was ejected from the game. (Nick Wass/Associated Press)

Washington Nationals pitcher Jordan Zimmermann was in the midst of a one-hit gem Sunday afternoon, mixing his 94-mph fastball with a biting curveball as well as he ever has since his debut in the big leagues in April 2009.

But even in the best of outings, the 6-foot-2 right-hander, who turns 25 on Monday, still has moments when his relative inexperience gets the best of him. One of those moments, combined with Washington’s season-long struggle at the plate, ended up costing the Nationals in a 2-1 loss to Baltimore before 33,626 fans at Camden Yards.

With the Nationals up 1-0 heading into the bottom of the seventh, Zimmermann threw a hanging curveball over the heart of the plate on an 0-2 count to Orioles slugger Vladimir Guerrero, and Baltimore’s designated hitter promptly lined a two-run homer that landed three rows into the left field bleachers.

Zimmermann never did make it out of the seventh inning, which only made his masterful work through six — he struck out five — that much more difficult to stomach. Zimmermann, whose previous scheduled start was skipped when last Tuesday’s game against Pittsburgh was postponed, finished the afternoon having given up four hits and two earned runs on 94 pitches.

“I had him 0-2 and I tried to bounce a curveball and I left it up a little bit,” Zimmermann said. “I thought I did a pretty good job tonight, but just that one mistake.”

Guerrero’s long drive was all the Orioles would need with Washington’s offense stymied again. The Nationals left nine men on base Sunday, but had no runners in scoring position after the fourth inning.

The loss, coupled with Saturday’s 8-3 defeat to the Orioles, means Washington hasn’t won an interleague series since June 16-18, 2009.

Manager Jim Riggleman’s day ended much earlier than Zimmermann’s. He was thrown out of Sunday’s affair two pitches into the first inning by home plate umpire Todd Tichenor, after leadoff man Roger Bernadina was called out when Tichenor overturned what appeared to be a bunt single.

Tichenor ruled that Bernadina had stepped on home plate when he bunted, a decision that brought Riggleman out of the dugout for a heated argument less than a minute after the day’s first pitch. The play proved costly when the next hitter, shortstop Ian Desmond, smoked a bases-empty double to center field.

“I didn’t really have an argument. I just was irritated the call was made,” Riggleman said. “The call was right, but it’s a call you just never see get made.”

The Nationals threatened Orioles starting pitcher Chris Tillman in the third inning when Alex Cora walked and Bernadina hit a double off the right field scoreboard. But Cora was unable to score from first base on the extra-base hit, and with men on second and third base and just one out, both Desmond and left fielder Laynce Nix struck out swinging to end the inning.

Washington finally got on the board in its next at-bat. The Nationals loaded the bases with one out after Jayson Werth hit a leadoff double in the top of the fourth. Tillman then walked designated hitter Wilson Ramos and hit second baseman Danny Espinosa in the lower back.

Catcher Ivan Rodriguez’s foulout down the right field line wasn’t deep enough to score Werth from third. But Cora, the No. 9 hitter, slapped a groundball up the middle that bounced off the pitcher’s mound and Orioles third baseman Mark Reynolds couldn’t corral it, giving Washington a 1-0 lead.

The Nationals could have done some significant damage, but outfielder Nick Markakis caught a long fly ball by Bernadina on the right field warning track, ending what amounted to a rally on this day.

For a time, it seemed that lone run would be enough with Zimmermann cruising. But with their offense still sputtering, the Nationals learned once again that all it takes is a hanging curveball to turn masterpieces into misery.

“We’ve been inconsistent and we know one day it’s just gonna hit us and we’re gonna take off,” said bench coach John McLaren, who took over for Riggleman after his ejection. “That day’s coming. We just haven’t gotten there yet.”