Tavares, who missed practice Saturday with an illness that limited his ice time, scored on a sensational move in the third round of the shootout to give the Islanders a 3-2 victory against the Ottawa Senators on Sunday.
The win was the Islanders' first in four games on a seven-game homestand.
The Islanders' star center played 19:09 in regulation and overtime. He said afterward he was about "70 or 80 percent." Coach Jack Capuano said that estimate might have been generous.
"I don't even know if he was 80 percent," Capuano said after the Islanders ended a three-game losing streak with their second victory in as many shootouts this season. "He was fighting it tonight. But at the percentage he was at, he was still one of the best players on the ice, and he made a great move in the shootout for us.
"He didn't feel good, but we knew he was going to play."
Tavares said he knew he had to husband his energy.
"The biggest thing for me is that I didn't feel like I had that extra gear at times," he said. "I tried to keep my shifts short to stay as fresh as I could. I tried to keep my energy as high as I could."
Nielsen started the tiebreaker by snapping a wrist shot past Lehner. Evgeni Nabokov stopped both Ottawa shooters, Jakob Silfverberg and Daniel Alfredsson, before Tavares' clinching goal.
“I didn’t think he was expecting the move I wanted to do,” Tavares said of the highlight-reel play. “I just tried to sell the shot to try and make him commit so that I could go back to my backhand.”
'Ottawa coach Paul MacLean was impressed.
"He's turning into a world-class player," MacLean said of Tavares.
Told that Tavares was under the weather, MacLean added with a laugh, "I hope he's sick every time we play him."
Ironically, it was the Islanders' first win in the nine games this season in which Tavares didn't have a point. It was their third win in 12 games at Nassau Coliseum this season.
"I told them yesterday that I'm tired of hearing about it," Capuano said. "I'm tired of listening about our home record, how we play at home. We have one singular focus, and that is to win hockey games."
It was a disappointing outcome for the Senators, who allowed Michael Grabner's go-ahead goal with 3:29 left in regulation but earned a point when Patrick Wiercioch's power-play slapper hit the stick of Islanders defenseman Travis Hamonic and went past Nabokov.
"Frustration is a useless emotion to me," MacLean said after his team's third game in less than 72 hours. "They gave a good account of themselves. We'd have liked to win the game, but they were a little bit better in the shootout than we were."
Grabner, one of the NHL's fastest players, jumped off the bench on a line change and raced into the Ottawa zone, where he picked up an errant clearing pass by Kaspars Daugavins and rifled a shot from the high slot past Lehner to put the Islanders in front.
Ottawa tied the game with 1:09 remaining when Wiercioch's straightaway slap shot from just inside the blue line sailed through a tangle of bodies and past Nabokov.
"We're glad to get the point," Wiercioch said. "For most of the game, we were just trying to keep it close and play how we play, where we can take two [points] late in the game or in overtime or the shootout."
Lehner, a rookie making his second start of the season, made 33 saves for the Senators. Nabokov stopped 29 shots for the Islanders.
The Islanders jumped out to a 1-0 lead at 7:48 when Brad Boyes scored his fifth of the season, snapping a shot from well inside the left circle off Lehner's glove and under the crossbar. Tavares started the play with a takeaway behind his own net; Boyes finished it by converting a pass from Matt Moulson.
Lehner made the best of his 10 first-period saves three minutes later when he used his pads to stop Tavares on a breakaway. The busiest player on either team was Islanders forward Matt Martin, who was credited with eight hits in the opening 20 minutes.
Both sides had chances in the early stages of the second period before Ottawa tied the game at 8:36. Chris Neil tried to pass the puck into the slot, but Hamonic got his stick on it. It deflected right to Mika Zibanejad, who quickly whipped a 15-footer behind Nabokov for his fourth of the season.
Ottawa had two excellent chances to break the tie on its first power play after Colin McDonald was called for hooking at 15:31. Alfredsson bulled his way from the right corner to the net, only to have his backhander denied by Nabokov, who got a break a few seconds later when a right-point blast by Wiercioch rang the left post.
The Senators had the better of play for the first half of the third period, but Nabokov preserved the tie with 11:40 to go when he stopped Colin Greening on a rebound try after making saves on Alfredsson and Wiercioch seconds earlier.