TAMPA, Fla. -- The Washington Capitals became the first team to clinch a playoff spot this season.
T.J. Oshie registered his third career hat trick, his first with Washington, and added an assist. Nicklas Backstrom had four assists for the Capitals.
Braden Holtby stopped 26 shots as the Capitals won for just the second time in the past seven games but reached 100 points for the third consecutive season.
"It feels good," Oshie said of clinching a playoff berth. "It's one thing on the checklist we wanted to cross off. We got there. Most important thing for us, we want to be playing at the very elite level, at our top level entering the playoffs. That's our goal."
Nikita Kucherov had two goals and Alex Killorn scored for the first time in a month for Tampa Bay, which lost in regulation in consecutive games for the first time since Jan. 31-Feb. 2. Andrei Vasilevskiy finished with 30 saves.
"It's tough because our identity here is we have to keep the puck out of the net, that's how we are going to keep winning," Lightning coach Jon Cooper said. "Back to back giving up five (goals) is tough, it's hard to win that way."
Washington looked to set the tone early and grabbed the lead on the first power-play chance of the game as Marcus Johansson fed Oshie in front to finish off a crisp passing play 3:44 into the game.
Oshie notched his second goal of the game, finishing off a two-on-one rush by converting a pass from Alex Ovechkin to give Washington a 2-0 lead at 11:52.
"We started off good and came out with good speed to get a 2-0 lead," Backstrom said.
Tampa Bay answered on its first power-play chance after Victor Hedman sent a puck into traffic that eventually found Kucherov near the right circle, where he roofed a shot over Holtby at 15:52 for his 32nd goal of the season and 100th of his career.
Killorn tied the score 16 seconds later as he cut from the right circle into the slot area and was tripped up but managed to fire off a shot as he was falling to the ice. His 17th goal of the season and his first since Feb. 4 pulled Tampa Bay even.
"We gave ourselves a chance, for sure, after building ourselves a 2-0 hole, you like your chances the rest of the way," Lightning defenseman Braydon Coburn said.
Williams broke the tie when he deflected a Matt Niskanen shot at 3:14 of the third period 13 seconds after a Washington power play expired.
"I thought we got stronger as the game went on," Washington coach Barry Trotz said.
The Capitals all but put the game out of reach when Carlson batted a puck out of the air during a delayed penalty call to open a two-goal lead.
"We started turning pucks over. We were a little bit on our heels," Lightning defenseman Victor Hedman said. "Obviously not the way we wanted to finish the game. We've got to learn from that.
"We can't afford any more of these kinds of games. This is not the time of year we can give away games."
Kucherov scored his second goal of the game with 1:30 left before Oshie found an empty net with 44 seconds left to complete the hat trick, flipping a puck up in the air from the neutral zone that slid in to the vacated net.
"I've been working pretty hard, but like you saw tonight a lot of these are hats off to my linemates," Oshie said. "We've got a great team here. I've been fortunate enough to be at the end of the score sheet a few things, but more importantly we've got our game back to where it needs to be."
-- Lightning LW Ondrej Palat and D Jake Dotchin, both of whom missed practice on Friday, were in the lineup for Tampa Bay.
-- Lightning RW Joel Vermin was called up from Syracuse of the American Hockey League on an emergency basis Saturday morning and was scratched.
-- Injured Tampa Bay C Vladislav Namestnikov (lower body) participated in an optional morning skate in a no-contact jersey but missed his third consecutive game.
-- Washington LW Marcus Johansson left the game midway through the first period because of illness and did not return.
-- The Capitals scratched D Nate Schmidt and D Taylor Chorney.
-- Washington's T.J. Oshie has 10 goals and 19 points in 14 career games against Tampa Bay.