Fisher scored 3:17 into overtime and Ottawa extended Tampa Bay's losing streak to a season-high seven games by beating the Lightning 4-3 on Saturday night.
The center put a shot from the left side past Johan Holmqvist for his 15th goal, giving the Senators their second straight win following consecutive losses to Washington.
"I just split their D along the boards and cut to the net and put it through his legs," Fisher said.
Dany Heatley scored his 25th goal 4:26 into the third period to get the Senators even at 3.
The Lightning are 0-5-2 since defeating Toronto at home on Dec. 20.
Holmqvist's clearing attempt behind the net struck teammate Kyle Wanvig's discarded stick in the left corner. Ottawa's Jason Spezza recovered the puck and sent a centering pass to Heatley, who beat Holmqvist with a wrist shot from the slot.
"That's the way it's going right now for us, but don't pity us. We can't have any self-pity," Lightning coach John Tortorella said. "We have to just keep on trying to find a way to complete a game and not just get one point, get the two points because as the games keep on rolling by here one point is not enough."
Wanvig, recalled from Norfolk (AHL) earlier in the day, made his season debut. He dropped his stick while he was checking Spezza earlier in the play that led to the tying goal.
"I was going to the net and he hooked me, and then he knew that he hooked so he let go of his stick and it just kind of drifted into the corner," Spezza said. "I started yelling at the ref, and all of a sudden the puck was there and I knew Heater was back there. That was the hockey gods helping out."
Randy Robitaille scored twice for the Senators, who won 5-3 in Buffalo on Friday.
Ray Emery made his second straight start and stopped 16 shots.
St. Louis opened the scoring with his 16th goal 5:43 in before Lecavalier netted his team-leading 28th goal after making a sensational stick-handling play.
Lecavalier, who leads the NHL with 62 points, received defenseman Paul Ranger's pass behind him and pushed the puck forward between his legs. He then put a backhanded shot past Emery at 11:17.
Holmqvist made 23 saves, and Jan Hlavac also scored for Tampa Bay, which hasn't won a road game in regulation since Nov. 10.
The Lightning are a league-worst 3-14-2 on the road.
Referees Kelly Sutherland and Don Van Massenhoven called only two penalties, both against Tampa Bay in the second period.
"I asked them between the second and third period, I let them know that we hadn't had any power plays yet and they told me that they haven't taken any penalties," Tortorella said. "I don't think I've been in a game in a long time where we haven't had that opportunity, but that's the way they called it."
Robitaille began Ottawa's comeback with 38.2 seconds remaining in the first when he beat Holmqvist to cut the deficit to 2-1.
Hlavac made it 3-1 with his sixth of the season 3:38 into the second, but Robitaille made it a one-goal game with his second of the night and sixth overall at 16:07.
Holmqvist started for the second time in eight games. He is 0-4-2 in his last six appearances. ... Lightning C Nick Tarnasky was a healthy scratch. ... With the Senators reaching the midway point of the regular season, Heatley is on pace for his third straight 50-goal season. He has scored 50 goals in each of his first two seasons with Ottawa, reaching the mark in the final regular-season game both times.