BALTIMORE – Jeremy Guthrie of the Baltimore Orioles was doing an outstanding job of matching zero for zero with Jon Lester of the Boston Red Sox as the game entered the seventh inning Saturday night.

And then, with one forceful swing, Kevin Youkilis tilted the momentum in favor of the Red Sox.

Youkilis ended the scoreless duel with his 11th home run, and Boston beat Baltimore 8-2 to extend the Orioles’ losing streak to 10 games.

Lester (7-2) gave up four hits and three walks in 61/3 innings to win his seventh straight decision, matching the longest unbeaten run of his career.

He’s 11-0 with a 2.06 ERA against the Orioles.

Guthrie (3-6) was nearly as effective. But after allowing only two hits through six innings, his first pitch of the seventh was driven by Youkilis into the left-field seats.

“I’m trying to get ahead, trying to throw a good two-seamer down,” Guthrie said.

“The last time I checked, he was a two-time All-Star, and he’s paid over $9 million to hit a home run. He’s as good as anybody. It just happened.”

Youkilis said, “I just hit a fastball. He pitched well; we’re very fortunate to get out with a win by grinding out at-bats.”

Boston added a run against Guthrie in the eighth when Josh Reddick tripled and scored on a single by Marco Scutaro.

The Red Sox pulled away in the ninth, using two-run doubles by pinch-hitter Darnell McDonald, Bill Hall and Youkilis to go up 8-0.

Baltimore ended a run of 19 straight scoreless innings in the bottom half when Scott Moore hit a two-run double, but that was perhaps the lone flaw in Boston’s seventh straight road win — its longest such run since June 2005.

The punchless Orioles have been outscored 19-2 in two games under interim manager Juan Samuel. Baltimore has scored only 16 runs during its season-high 10-game skid, including 10 over the last eight games.

“We’re just trying to stay positive,” Samuel said, “and the clubhouse is very positive right now.”

After Youkilis homered in the seventh, Baltimore threatened in the bottom half. Lester walked the bases loaded with one out, but Daniel Bard retired pinch-hitter Luke Scott on a short fly to center and got Corey Patterson to foul out.

“If we score some runs in that inning when we had the bases loaded, and we had the right guys up there, I think it was a different ballgame,” Samuel said.

Lester pitched a fine game, but was angry at the way he finished after Youkilis gave Boston the lead.

“Youk did that, put us on top, and that’s huge,” Lester said. “I’ve got to do a better job in the seventh inning.

“I got the first guy out but it got away from me a little bit. I walked three guys in a row. I’m not happy. It’s obviously not what I wanted to do.”

For a while, it was a tight game.

The Red Sox had runners at second and third with two outs in the first inning before Victor Martinez struck out, the first of 15 straight batters retired by Guthrie.

In the bottom of the fifth, Baltimore used two singles and a bunt to put runners at second and third with one out. But Cesar Izturis hit a popup and Patterson grounded out.