Alex Ovechkin scored his 50th and 51st goals for the Capitals to become the third player in NHL history with at least eight 50-goal seasons; Wayne Gretzky (Edmonton Oilers, Los Angeles Kings) and Mike Bossy (New York Islanders) each had nine.
Washington (47-24-8) leads New York by three points for first place. The Capitals and Islanders, who defeated the Buffalo Sabres 5-1, each have three games remaining.
[WATCH: All Capitals vs. Lightning highlights]
Ovechkin scored his 50th goal at 14:35 of the third period to give Washington a 5-2 lead. He made it 6-2 with a power-play goal from above the left face-off circle at 16:15.
"Obviously you want to score as early as possible, but, you know, I had chances before, but I knew it was going eventually come, especially when the guys know I had 49, so they try to find me all the time," Ovechkin said. "So thanks to them."
Video: WSH@TBL: Ovechkin scores 50th goal for eighth time
T.J. Oshie and Nicklas Backstrom each had two goals and an assist for the Capitals, who have won four games in a row. Braden Holtby made 25 saves.
"They've got a good team, they are the best team in the League right now," Washington forward Tom Wilson said. "They have had our number this year so far (Tampa Bay won the first two games), so we wanted to come in tonight and just feel good at our game, and I think we did that tonight."
J.T. Miller, Tyler Johnson and Nikita Kucherov scored for the Lightning (59-15-4), who have four games remaining to try to become the third team in NHL history to win 60 games (Detroit Red Wings, 62-13 with seven ties in 1995-96; Montreal Canadiens 60-8 with 12 ties in 1976-77). Andrei Vasilevskiy made 29 saves.
"We just didn't play well enough to win," Tampa Bay captain Steven Stamkos said. "Whether we're playing Washington, [the] Boston [Bruins], [the] Toronto [Maple Leafs], whatever, you play the way we did the first half of the game, you're not going to win."
Lightning defenseman Victor Hedman did not play after the first period because of an upper-body injury, and defenseman Erik Cernak did not return after a fight with Wilson at 12:13 of the third.
"Hedman got hit in a, kind of, tough fashion, so we'll re-evaluate him," coach Jon Cooper said. "But [Cernak] is going to be OK."
Backstrom gave the Capitals a 1-0 lead at 7:24 of the first period with a one-timer from the slot off a pass from Wilson.
Video: WSH@TBL: Backstrom buries feed from Wilson
Oshie made it 2-0 at 10:45 when he knocked in the rebound of a Nick Jensen shot. He scored on the power play to give the Washington a 3-0 lead at 16:00 when he redirected a John Carlson shot.
"I think [against] Tampa in general and a lot of the games going forward here, that first goal is going to be important," Oshie said. "To get that momentum shift on to your side, especially on the road, early in the game is only going to help you."
Video: WSH@TBL: Oshie deflects home second goal of the game
Backstrom made it 4-0 at 7:53 second period with his second goal of the night.
Miller redirected a Mikhail Sergachev shot to make it 4-1 at 17:25.
Johnson scored unassisted to make it 4-2 at 3:36 of the third period.
After Ovechkin's two goals, Kucherov made it 6-3 when he scored on the power play with 28 seconds remaining. It was his 39th goal.
Video: WSH@TBL: Kucherov blasts power-play goal in 3rd
They said it
"We came to work, we knew we had to work. We knew we had to work away from the puck against them. They're so dangerous on line rushes, they're so dangerous in the offensive zone, they're where they're at for a reason. We knew we had to be on top of our game if we were going to have a chance tonight." -- Capitals coach Todd Reirden
"In the [Stanley Cup] Playoffs, you can't afford to take five minutes off or 10 minutes, let alone a period. We were not ready to go there for the first period, and it kills your chances, for sure." -- Lightning defenseman Ryan McDonagh
"We had plenty of chances. But for me, the past few games here we are giving up too much, and that has to get better in our game. We end up losing this game, but by no means did it need to be six against us." -- Lightning coach Jon Cooper
Need to know
Ovechkin (33 years, 194 days) is the third-oldest player in NHL history to score 50 goals in a season behind Johnny Bucyk (35 years, 308 days in 1970-71) and Jaromir Jagr (34 years, 37 days in 2005-06). … Ovechkin passed Brendan Shanahan for 13th in goals in NHL history. They were tied at 656 entering Saturday. … Ovechkin's second goal was his 247th on the power play, moving him past Phil Esposito and into a tie with Luc Robitaille for fourth in NHL history. … Oshie had six points (three goals, three assists) in three games against the Lightning this season. … Kucherov leads the NHL with 122 points, eight ahead of Connor McDavid of the Edmonton Oilers. ... The Capitals and Lightning each have clinched a playoff berth.
Capitals: At the Florida Panthers on Monday (7 p.m.; TVAS, FS-F, NBCSWA, NHL.TV)
Lightning: At the Ottawa Senators on Monday (7:30 p.m.; TSN5, RDS, SUN, NHL.TV)
Video: Ovechkin surpasses 50 goals in Caps' 6-3 victory