Thornton spent most of his offseason strengthening his legs, particularly his quads.
"It was pretty much just to get my knee strong again," Thornton said. "But they feel real strong. I feel a lot of pop out there. They're probably as strong as they've ever been just because I had to rehab that knee so much."
Sharks center Logan Couture said he wasn't surprised to see Thornton fully healthy Friday, not after seeing him return during the playoffs with two torn knee ligaments.
"I couldn't believe it at the time," Couture said. "I believe he was skating three days after the injury. That blows my mind, and all of the other hockey players around the League who I talk to are still shocked by it. Guys with the same injury are out for six weeks, seven weeks."
Thornton said it was just business as usual for him Friday, other than the fact that Patrick Marleau, who was his longtime teammate, wasn't on the ice. The forward left the Sharks as a free agent, signing a three-year contract with the Toronto Maple Leafs on July 2 after 19 seasons with San Jose.
"I've spent a lot of years with him," said Thornton, who played 12 seasons with Marleau. "It is kind of strange. It's his (38th) birthday today too. It's a little weird, but he's going to do great up in Toronto.
"He's still a friend. We're going to miss him, but he's in a different uniform and we're in the same one."
Marleau is the Sharks leader in several categories, including goals (508), points (1,082) and game-winning goals (98). He's coming off a 27-goal season, and the Sharks will try to replace that production.
"A lot of guys, you know myself, everybody has to step up a little bit, score a couple more goals this year," Thornton said. "I think we've got some good young players that can accept a bigger role on the team. It's going to be by committee, how you replace a guy like that."
Though Wilson said the Sharks love Marleau and have great respect for him, he's confident San Jose can make up for his departure.
"He's a 38-year-old player," Wilson said. "We have a really good hockey team. Potentially the easiest position to play and integrate into this league is the wing position when you're strong down the middle. We have some players who are going to get extra ice time or play on that first power-play unit.
"We think it's just a natural growth or opportunity. Any team that's successful in this league gets contributions from everybody."