Tomas Vokoun made 22 saves against his former team to help the Capitals move within two points of the division-leading Panthers.
"A big game, I was a little nervous, too," Vokoun said. "It's not easy playing back where I played last year, and we're chasing them. But I settled in and obviously a great finish by our guys."
Washington had been winless (0-2-1) since its 4-0 home win over Florida on February 7. The Panthers had gone 3-1 since that loss to the Capitals last week.
Florida drew first blood with just 39.8 seconds left in the first period. Stephen Weiss carried the puck around the net and passed to Fleischmann, whose one-timer from the left circle beat Vokoun.
The Capitals outshot the Panthers by a 13-4 margin in a scoreless middle stanza.
Ovechkin finally put Washington on the board at the 4:47 mark of the third. With the Capitals on the power play, a scramble for the puck took place in front of Theodore. The disc ended up on Ovechkin's stick and the Russian star found the back of the net.
Semin put the Capitals in front with 6:33 to play, as his slapper from the top of the right circle beat Theodore. The puck deflected of the stick of Florida's Sean Bergenheim and changed direction before getting past the Panthers' netminder.
Vokoun made seven saves in the final frame to preserve the victory.
"When you have a lead you at least want a point out of it, but we found a way to lose tonight...It's a regular-season game, obviously, two teams trying to get to the playoffs, it was definitely an intense game," Panthers forward Kris Versteeg said.
Washington has taken three of five from Florida this season...Ovechkin has amassed 50 points (23 goals, 27 assists) in 42 career games against the Panthers...Washington started a four-game road trip on Friday...The Capitals went 1-for-6 on the power play, while the Panthers went 0-for-3.