Orioles 6, Red Sox 4: Chris Davis' RBI single in the top of the 13th inning snapped a tie and led Baltimore to victory in the opener of a three-game series.
Mark Reynolds homered and drove in two runs for the Orioles, who have won nine of their last 11 games.
Matt Wieters hit a one-out single off Franklin Morales (0-1) and scored two batters later on Davis' single to right field. Reynolds followed with a sacrifice fly off Scott Atchison, giving Baltimore a two-run cushion.
Troy Patton (1-0) tossed two scoreless innings and Jim Johnson worked a perfect 13th for his eighth save in as many opportunities.
Boston took a 1-0 lead in the second when Cody Ross led off with a triple and scored later in the inning on a passed ball by Ronny Paulino. But Baltimore answered quickly as Reynolds opened the third with his first homer of the season.
The Red Sox scored two in the bottom half for a 3-1 advantage. However, Wieters plated Adam Jones with a triple and scored on Wilson Betemit's single in the fourth to again knot the contest.
Dustin Pedroia hit an RBI single in the fifth and Reynolds scored on a double-play grounder in the seventh to forge the third tie of the night.
Adrian Gonzalez collected three hits and scored a run for the Red Sox, who have lost four of five.
GAME NOTEBOOK: Boston starter Jon Lester allowed three runs over six innings but received a no-decision, remaining unbeaten against Baltimore. The left-hander is 14-0 in 19 career starts vs. the Orioles. ... The Red Sox fell to 1-3 on their six-game homestand while the Orioles improved to 3-1 on their six-game road trip. ... Baltimore starter Wei-Yin Chen allowed more than two earned runs (three) for the first time in five major-league outings. ... Boston has scored more than three runs in only three of its 14 losses.