BOSTON — Tim Thomas had every reason to be confident heading into overtime of Game 7 in Boston’s first-round playoff series against the Washington Capitals.

After all, the Bruins won three seventh games in last year’s run to the Stanley Cup title.

“I’m probably in shock,” Thomas said Wednesday night after Joel Ward beat him at 2:57 of overtime to lead the Capitals to a 2-1 victory over Boston and give Washington a spot in the Eastern Conference semifinals. “I really believed that we were going to win.”

Thomas made 26 saves for Boston, which was hoping to become the first team to repeat as Stanley Cup champions since the Detroit Red Wings in 1997-98. Tyler Seguin scored the Bruins’ goal as rookie Braden Holtby stopped 31 shots for Washington in the seventh one-goal game of the series.

Matt Hendricks also scored for the Capitals, who barely made the playoffs this year after finishing atop the Eastern Conference in each of the previous two seasons — and winning just one postseason series combined.

It was the first time in NHL history that a playoff series had seven games determined by one goal. Four of the games went to overtime, and two others were decided with less than two minutes left in regulation.

Hendricks scored midway through the first period, and Seguin tied it in the second. It stayed that way through the third, with Washington killing off a penalty in the final 3 minutes to send the game into overtime.

Patrice Bergeron had a chance to win it in the first minute of the extra period, but he couldn’t get off a solid shot from Holtby’s right. Two minutes later, the Capitals broke into the Boston zone with former Bruin Mike Knuble leading a 2-on-1.

Knuble shot, and Thomas left the rebound out where Ward could reach it with his backhander.

The building fell silent as the Capitals celebrated just their third postseason series win since a run to the Stanley Cup finals in 1998. Some fans littered the ice with debris, but the Bruins waited patiently for the postgame handshake.

Thomas, the reigning Vezina and Conn Smythe Trophy winner, gave Holtby a tap on the shoulder and said, “Great job, kid.”

The Capitals earned more than 100 points in the previous three seasons, leading the NHL with 121 in 2010, but had little to show for it once the postseason started. They won just two series in three years and have not made it out of the East semis since 1998, when they made it to the Stanley Cup finals but were swept by Detroit.

This year’s regular season wasn’t as successful, with coach Bruce Boudreau fired in November and replaced by Dale Hunter.

Waiting until the final games of the season to clinch a playoff berth and getting bumped up to a No. 7 seed on the final day, the Capitals won three times in Boston — they also won Games 2 and 5. Their second-round opponent won’t be determined until after Game 7s Thursday night between Ottawa and the New York Rangers, and Florida and New Jersey.

The Bruins needed an unprecedented three Game 7s to win the Cup last year, including the 4-0 victory over Vancouver that gave the Original Six franchise its first title since 1972. Thomas also had a shutout in the Game 7 win over Tampa Bay in the Eastern Conference finals.

But that streak ended midway through the first period when Carlson shot from right point and Hendricks tipped it past Thomas’ right shoulder to give the Capitals a 1-0 lead.

The Bruins tied it with 5:33 left in the second when Holtby kicked Andrew Ference’s shot wide to his left to Johnny Boychuk, whose shot trickled through the goalie’s pads and into the crease. Seguin dove for it, with two defenders crashing on top of him, and swiped it into the net to make it 1-1.