Dave Bolland and Niklas Hjalmarsson also scored for the Blackhawks, who remain the league's only team without a regulation loss. Jamal Mayers had two assists and backup Ray Emery made 27 saves for his fourth win in four starts this season.
"Everything was good tonight," Toews said. "The penalty kill was huge in the first period, with the big 4-minute penalty kill we had to kill. Big goal by Bolly after that, and I think that just set the tone for the night with that penalty kill."
The Blackhawks were forced to play a man short after Toews and Thornton got into it with about 4½ minutes left in the first. The star centers locked up for a minute before Thornton dragged Toews to the ice and the linesmen skated in.
"He asked me to fight and I was kind of shocked," said Thornton, who became San Jose's captain in 2010. "I said, 'Sure.' That's how it went. He'd hit me from behind, but it wasn't anything serious. Then he just asked me to fight, so I obliged.
"Hopefully, the guys could rally after it. They didn't, but we'll be back. What do you do? They want to fight, so sure, I'll fight."
Toews received 2 minutes for boarding, 2 minutes for unsportsmanlike conduct and a 5-minute fighting major. Thornton got 5 minutes for fighting. But the Sharks were unable to take advantage of the resulting power play.
"There's always little things, players that are going to be chipping at you and trying to get under your skin," Toews said. "I think the rest is history right now. Like I said, I felt it was something I needed to do, stand up for myself, and I did it and glad I did it. I think it helped our team."
San Jose has dropped seven in a row after getting off to a 7-0 start. Three of the losses have come in a shootout or overtime.
"Tonight, let's face it: One team was fast, moved pucks, attacked," San Jose coach Todd McLellan said. "Another team bobbled pucks and misplayed them and was slow. Simple as that. We haven't been that way all seven games."
Each of the Blackhawks' first three goals came on rebounds, including an ugly gaffe by Niemi in the second period.
Bolland skated toward the goal in the final minute of the first and let loose with a low shot that Niemi stopped with his right leg. But the deflection trickled off Niemi's left arm and Bolland tucked it into the open net while the veteran goalie frantically tried to find the puck.
The Blackhawks increased the lead to 2-0 at 3:05 in the second, taking advantage when Niemi whiffed on Kruger's bouncing shot from just inside center ice. The puck bounced off the inside of the right post and slid across to the left side, where Kruger shot it into the open net as Niemi turned around in a full circle.
After Kennedy got the Sharks within one, Jason Demers was sent off for roughing against Patrick Sharp and the Blackhawks made the most of the power-play opportunity. Shaw's first rebound try was stopped by Niemi, but his second attempt popped up and went in off the goalie's back to make it 3-1 with 5:20 left in the period.
The Blackhawks (11-0-3) outshot the Sharks 17-8 in the second.
"That gritty attitude is contagious," said Emery, pointing to Toews' fight in the first. "We have a very skilled team and guys who can put the puck in the net, but when guys are willing to fight for it, it adds to that other element."
The lone bright spot for San Jose was Kennedy's first goal since Oct. 22, 2011, against the New York Islanders. The 26-year-old forward took a pass from Logan Couture on the left side, skated in on Emery and went to his backhand for the pretty score 5 minutes into the second.
The NHL record for a season-opening point streak by one team is 16 games by Anaheim in 2006-07. ... It was believed to be the first fight for Toews since December 2011, and the first skirmish for Thornton in about a year. ... G Corey Crawford is day-to-day with an upper-body injury, so the Blackhawks promoted Henrik Karlsson from Rockford of the American Hockey League to serve as the backup to Emery. ... Niemi helped the Blackhawks win the Stanley Cup in 2010.