The Washington Capitals weren’t without their imperfections in the middle of the second period Friday night. But they still held a one-goal lead over the Tampa Bay Lightning in the opening contest of their Eastern Conference semifinal series and appeared poised to strike again.

In less than four minutes, though, the balance shifted to their foe. Steve Downie scored on a shot that went off Washington defenseman Scott Hannan’s stick, and Steven Stamkos recorded a power-play goal that stood as the game winner in a 4-2 Tampa Bay victory at Verizon Center.

“I thought the game plan, we had gotten away from, but I thought we were in control of the game until the Downie goal and that gave them life,” Coach Bruce Boudreau said. “It was reverting back to an older day. When you start playing chance for chance — they’re a pretty skilled group over there as well — and when you’re behind, it’s something you can’t afford to do.”

The Capitals tried to mount a comeback but couldn’t gain any traction against a Tampa Bay squad content to sit back, clog up the shooting lanes and smother an ineffective power play. Washington went 0 for 5 on the man advantage, including two chances in the third period, in an exercise in futility that led to the first loss at home this postseason. The Capitals are 3 for 23 in the postseason on the power play.

Dwayne Roloson made 26 saves and Sean Bergenheim and Dominic Moore scored the first and last goals of the game, respectively, but it was that brief stretch at the end of the second period that frayed the Capitals’ cause most.

Alexander Semin draws the Capitals even at 1-1 with this first period goal. (Toni L. Sandys/The Washington Post)

“That’s exactly how we expected them to play and we had a really good game plan,” said Eric Fehr, who scored one of Washington’s two goals. “We had it going for a while and we kind of gave up on it. Things didn’t go well after that.”

Washington, well rested since its last game five days earlier, missed out on an opportunity to jump on Tampa Bay, which wrapped up its first-round series less than 48 hours earlier. But there may have been a more damaging loss to the Capitals in Game 1.

Rookie defenseman John Carlson went off the ice in considerable pain with 5 minutes 23 seconds remaining in the second period. He returned to the bench at the start of the third but only took two shifts, for a total of 36 seconds, the remainder of the game. Carlson, who slammed his stick in frustration after his final 16-second shift, is listed as day-to-day. “I’m hoping he can go Sunday,” Boudreau said.

Without Carlson in the mix, the Capitals were not only down to five defensemen when they tried to climb back into the game, but they were without one of their most offensive-minded blue-liners. Even when Carlson was a part of the power play, it didn’t experience much more success.

Out of the nine power-play opportunities in the contest — five for the Capitals, four for Tampa Bay — only the Lightning managed to score. Stamkos’s rebound shot with 31.6 seconds remaining in the middle period gave the Lightning the only lead it needed.

While Tampa Bay has grown confident in playing with the lead in these playoffs, arguably nothing proved more frustrating for Washington than the power play’s lack of bite. On the Capitals’ final man advantage with less than six minutes remaining, they were whistled offsides three times, unable to set up in the offensive zone.

The Lightning already smothered Pittsburgh’s power play in the first round, and with five more successful kills Friday night, it’s 39 of 40 in the playoffs.

“Our players put a lot of work into the details,” Lightning Coach Guy Boucher said of shutting down Washington’s power play. “They spend a lot of time watching video by themselves or in little groups on top of what we made them watch. . . . You look at their power play, they still had a lot of time of possession in our zone.”

Tampa Bay’s comeback erased a solid first half of the contest by Washington, save the opening five minutes, in which Bergenheim scored to establish a quick 1-0 lead for the visitors. Michal Neuvirth (20 saves) wouldn’t allow another goal in the early flurry, though, and Alexander Semin tied the game just past four minutes in with a shot that sneaked through Roloson’s legs.

At the start of the second period, Fehr scored from out in front after Jason Chimera won a race to the loose puck off a faceoff and sent a pass directly to the right wing to make it 2-1.

Washington came close to taking a larger lead, with both Roloson and the Lightning defense getting caught out of position on consecutive shifts. When the Capitals couldn’t move forward, they left the door open for Tampa Bay, which had three power-play opportunities in a less-than-12-minute span of the second period.

“When we get the lead, we didn’t play our game,” Capitals all-star winger Alex Ovechkin said. “I think we play too cute, and we took lots of penalties and that cost us. It’s over and we have to prepare for next game.”

Notes: Tampa Bay winger Simon Gagne left the game after being driven into the boards by Hannan, but he was able to leave the ice under his own power and was awake and alert for doctors’ evaluations. Gagne, along with Lightning defenseman Pavel Kubina, whose head was hit into the glass by Chimera’s elbow on a check in the second period that earned the Capitals winger a roughing penalty, will be re-evaluated tomorrow, Boucher said.