BALTIMORE -- In case it wasn't evident before opening day, we now know why the Baltimore Orioles added another slugger to a team that led the major leagues in home runs last season.

Nelson Cruz celebrated his Orioles debut by hitting a tiebreaking home run in the seventh inning, and Baltimore beat Jon Lester and the defending World Series champion Boston Red Sox 2-1 Monday.

Signed as a free agent in February, Cruz lined a solo shot into the left field seats to put the Orioles ahead. He hit 27 homers last year with Texas during a season marred by a 50-game suspension from Major League Baseball in the Biogenesis drug scandal.

Cruz joins a batting order that features Chris Davis, who led the majors in 2013 with 53 homers and 138 RBIs. Davis went 0 for 3 with two strikeouts as the cleanup hitter, but it didn't matter because the guy in the 5-hole picked up the slack.

"It takes some of the pressure off everybody in the lineup knowing that you can pass the baton and the next guy can pick it up," Davis said. "We have to know as hitters going into the game that we can be patient, we can wait for our pitch, knowing that the guy behind us can pick us up."

Cruz, meanwhile, couldn't have been more delighted. Not only did the Orioles provide him with a fresh start, but the fans chanted "Cruuuuuze!" every time his name was announced.

"It was really neat, it was special," he said. "I made the right call to come and be part of this organization, be part of this town."

Cruz wasn't the only player to enjoy a successful start with his new team.

After missing two seasons recovering from knee surgery, Boston center fielder Grady Sizemore marked his return from a 922-day absence with a second-inning single for his first hit since September 2011. The three-time All-Star for Cleveland later hit his first home run since July 15, 2011, which also came at Camden Yards.

"Today was very exciting," Sizemore said. "I couldn't wait to get to the ballpark."

Zach Britton (1-0) got the win with two scoreless innings of relief and Tommy Hunter got three outs for the save. Hunter earned the job as closer this spring after the Orioles dealt away Jim Johnson during the offseason.

The Red Sox got two on with one out in the ninth before World Series MVP David Ortiz hit a flyball and Jackie Bradley Jr. took a called third strike.

"It's life in baseball," Hunter said. "You've got to earn everything you get. One-run game to start the season off against the defending world champs."

Starting on the road for the fourth year in a row, the Red Sox got nine hits but stranded 12 and went 0 for 10 with runners in scoring position.

"We had a lot of opportunities, we had a lot of guys on," second baseman Dustin Pedroia said. "We just hit the ball right at people. It was one of those deals where we couldn't find a way to push runs across."

Making his fourth straight opening day start for Boston, Lester (0-1) gave up two runs, six hits and a walk in seven innings. He struck out eight.

Orioles starter Chris Tillman allowed one run, seven hits and a walk in five innings. The right-hander went 3-1 against Boston last year, but in this one he needed 104 pitches to get 15 outs.

Britton gave up a one-out double to Xander Bogaerts in the sixth and got three straight outs in the seventh, the only time all day that Boston went down in order.

Evan Meek walked two in the eighth before Brian Matusz retired A.J. Pierzynski on a comebacker.

Gametime temperature was announced at 60 degrees, but a stiff wind chilled many in the crowd of 46,685 -- the 23rd consecutive opening day sellout at Camden Yards.

Hopeful that the offseason acquisitions of Cruz and right-hander Ubaldo Jimenez will enable Baltimore to return to the playoffs after missing out last year, Orioles fans were out in force. Until recently, games against Boston often drew as many Red Sox backers as those rooting for the home team.

"Our crowd was legit," Hunter said. "That was awesome. I hope they come out like that every game for 80 more here."

The Orioles used a walk, a single by Matt Wieters and a double-play grounder to take a 1-0 lead in the second.

The Red Sox put runners at second and third with two outs in the third before Tillman struck out Mike Carp. In the fourth, Sizemore hit a 3-1 pitch over the right-field wall.

NOTES: Dustin Pedroia got Boston's first hit of the season, a one-out single in the first inning, to extend his hitting streak on opening day to eight games. ... After taking Tuesday off, the teams meet Wednesday night. John Lackey (13-5 lifetime against Baltimore) faces Jimenez in his Orioles debut. ... The Red Sox put RF Shane Victorino (hamstring) on the 15-day disabled list and recalled Bradley. ... Ortiz received his 12th opening day start at DH, tying Don Baylor's major league record at the position.

Nationals 9, Mets 7

NEW YORK (AP) -- Dillon Gee, Juan Lagares, Andrew Brown -- all the opening-day subs delivered for the New York Mets before a rare packed house at Citi Field.

They were one strike away from a perfect afternoon when that suspect bullpen blew it.

For the second time in three innings.

Denard Span hit a tying double with two outs in the ninth off closer Bobby Parnell, and the Washington Nationals rallied past the Mets 9-7 Monday to give Matt Williams a win in his first game as a major league manager.

"Just got to do a better job of coming out and shutting them down," New York manager Terry Collins said.

Anthony Rendon hit a three-run homer in the 10th and had a career-best four RBIs, twice rallying the Nationals in the late innings. Gee left with a 4-3 lead in the seventh but Washington tied it when relievers Carlos Torres and Scott Rice issued consecutive four-pitch walks, forcing in a run.

"We ran into some of that in spring training," Collins said. "Certainly it's got to be addressed at this level. If you are going to pitch here, you've got to throw strikes."

Ian Desmond put the Nationals in front for the first time with a sacrifice fly in the 10th. Rendon connected two batters later against former Washington pitcher John Lannan.

Stephen Strasburg struck out 10 over six innings in his third straight opening-day start. But he was outpitched most of the day by Gee, given the starting assignment because of injuries to Matt Harvey and Jonathon Niese.

Several fill-ins came through for New York, in fact. Lagares and Brown both homered after they were surprise additions to the lineup.

Aaron Barrett (1-0) struck out two in a perfect inning to win his major league debut with family in the stands.

Jeurys Familia (0-1) took the loss, hurt by Travis d'Arnaud's passed ball. David Wright hit a two-run homer in the 10th.

Lagares put New York ahead 5-4 with a leadoff homer in the eighth against Tyler Clippard, but the Mets couldn't hold it. Parnell failed in his first save opportunity since a neck injury that eventually required surgery sidelined him for the final two months last season.

"Bobby will be just fine," Wright said.

The Mets lost for only the third time in their last 23 season openers at home. They still own the best mark in the majors on opening day at 34-19.

New Yorkers woke up to an icy mix of sleet and snow flurries -- only fitting after the frigid winter much of the country endured. The temperature at first pitch was 44 degrees, with wind gusting to 25 mph, but it warmed in the afternoon.

Gee pitched in short sleeves and retired 15 in a row after Adam LaRoche's long two-run homer in the second.

Rendon, the new starter at second base, chased Gee with a two-out RBI double in the seventh. Washington tied it on the walks by Torres and Rice.

"That situation, I can't let that happen," Rice said.

Brown, a surprise starter in left field because newcomer Chris Young sat out with a mild right quadriceps strain, clubbed a three-run shot on a 94 mph fastball from Strasburg in the first inning.

Chris Young's injury also opened a spot for Lagares, who started in center. Eric Young Jr., normally the left fielder, shifted to second base to fill in for Daniel Murphy, who was in Florida with his pregnant wife awaiting the birth of their first child.

Eric Young had a sacrifice fly in the second.

"We got a great performance today by Dillon. Unfortunately, we couldn't get him a win," Wright said. "There's some things that we need to work on moving forward and that was pretty glaring in game one."

NOTES: One reliever who did get the job done was Jose Valverde, who struck out three over 1 1-3 perfect innings in his Mets debut. He fanned Ryan Zimmerman on a full-count pitch with the bases loaded to keep the score tied in the seventh and bolted excitedly off the mound. ... The Mets announced on Twitter that they have signed 40-year-old outfielder Bobby Abreu to a minor league contract. Abreu, a two-time All-Star who was released by Philadelphia late in spring training, did not play in the majors last season. He will report to Triple-A Las Vegas. ... After an off day Tuesday, Chris Young and Murphy are both expected back in the lineup Wednesday night, when RHP Bartolo Colon makes his Mets debut against LHP Gio Gonzalez. ... Cleanup hitter Curtis Granderson went 0 for 5 with three strikeouts in his first game for the Mets. ... Late slugger Ralph Kiner, a Hall of Famer and Mets broadcaster since the team's inception in 1962, was honored before the game. Kiner died in February. ... Harper was shaken up early when he got kicked in the forehead by Eric Young Jr. while breaking up a double play. Harper stayed down for a minute or two but stayed in the game.