Thornton's assist set up Patrick Marleau's winning score 7:07 into overtime and his second goal in two games sparked a comeback for San Jose, which beat the Detroit Red Wings 4-3 on Tuesday night and grabbed a 3-0 lead in the second-round series.
The sweet-passing center has been called "No Show Joe" by some for not producing in the playoffs.
"I don't know how this evolved, but you saw a very determined 19," Sharks coach Todd McLellan said, referring to Thornton by his jersey number.
Thornton, meanwhile, shrugged off what he had done.
"Business as usual," he said.
San Jose has been dogged for failing to get to the Western Conference finals since its longest run in 2004, but an impressive rally from a 3-1 deficit with 13 minutes left in regulation could prove to be pivotal for the franchise.
"Hopefully down the road we can look back and say, 'You know this was a turning point,'" defenseman Dan Boyle said. "But again, there's still too much work ahead of us."
Game 4 is Thursday night in Detroit, where the Red Wings will try to stay alive.
"They didn't have to win that one, they have to win the next one," McLellan said. "That's when it becomes a must win."
The Red Wings looked as though they had put themselves in a great position to win Game 3 -- leading 2-0 in the first and 3-1 in the third -- but they blew chances to pad the lead early, and goalie Jimmy Howard let them down late.
"It's a tough pill to swallow," Detroit coach Mike Babcock said.
The two-time defending Western Conference champions haven't been swept since 2003, in the first round by Anaheim, but they are determined to put up a fight.
"It feels bad, but it's still not over," said Howard, who made 29 saves. "You still can't stop believing."
Thornton started San Jose's comeback by scoring 6:42 into the third. Logan Couture tied it with 6:43 left, flicking a shot toward Howard from the right corner. Howard didn't press his body firmly against the post, and that allowed the tying goal to go off him and into the net.
"The coaches have been telling us to throw it at his feet all series," Couture said. "I waited for him to open up his pads."
After the puck went in, Howard tilted his head back and looked to the rafter-filled banners in disbelief.
"It's a tough one," Babcock said. "But the goalie picks you up a lot, too."
"That was a big wake-up call for us," McLellan said.
The Red Wings got started on time, as Babcock likes to say, when Tomas Holmstrom and Daniel Cleary scored in the first to give them a 2-0 lead. San Jose, though, pulled within a goal when Devin Setoguchi scored with 4 seconds left in the period.
Zetterberg restored Detroit's two-goal lead early in the second.
The Red Wings would've had a bigger lead in the third period if not for a disallowed goal in the first.
Zetterberg had a goal negated by video review because the puck went in off his left skate in what was ruled to be a distinct kicking motion.
"I wasn't kicking," he insisted.
Zetterberg was also stopped by Nabokov on a penalty shot -- awarded in the first when Couture made a rookie mistake of covering the puck in the crease with his glove.
"We had a chance to win this game, for sure," Babcock said. "We had lots of opportunities and it got away from us."
Detroit did have a review go its way in the first period when the puck went off Holmstrom's right skate and the on-ice ruling of a goal stood.
Thornton scored on a wraparound goal -- sending the puck off Pavel Datsyuk's stick and into his own net -- and made a perfect play on the odd-man rush to allow Marleau to score easily into an open net.
"He's a world-class passer," McLellan said. "He even fooled me. I thought he was going to shoot."
The crowd at Joe Louis Arena stood silently for a couple of minutes after the goal, knowing the Red Wings had fallen into a lot of trouble against top-seeded San Jose.
These are suddenly not the same-old Sharks.
"I don't think we were out to convince anybody. We knew we were a different team," McLellan said. "I guess what we have to do is prove that to the hockey world."
San Jose's Joe Pavelski, the first player since Mario Lemieux in 1992 to score more than once in three straight playoff games, had his five-game goal streak snapped. ... The Sharks had won three of nine playoff games in Detroit, losing the last two in the 2007 Western Conference semifinals, and just five of 35 matchups during the regular season.