Karlsson, 26, won a silver medal at the 2014 Sochi Olympics. He'll be a key figure on Team Sweden's defense, which includes Oliver Ekman-Larsson (Arizona Coyotes), Victor Hedman (Tampa Bay Lightning), Niklas Hjalmarsson (Chicago Blackhawks), Niklas Kronwall (Detroit Red Wings) and Anton Stralman (Lightning).
The eight-team tournament takes place at Air Canada Centre in Toronto from Sept. 17 to Oct. 1.
"Everybody in the regulation round is really good," Karlsson said. "I don't think there is a certain team [to beat]. If you look at the rosters, there's certain teams that are better looking on paper, maybe, from what every individual player has done throughout their career, but every team has a chance to win.
"It's a tournament format. A lot of things happen. It's not a playoff series. It's not a regular-season game. It's very different to what we're used to. Anything can happen. We can have a bad day and it can end quick."
Karlsson adjusted his usual offseason workout schedule to accommodate the World Cup, getting on the ice to skate with teammates and other professionals earlier than normal.
"We're going to play our first competitive game in early September here and put some more effort on being on the ice and trying to get the legs going out there," he said. "Unfortunately it's been taking some time from the gym, but hopefully it will be for the better. It's definitely changed things up. It's always nice when you change sometimes. It's been a good summer. It feels good right now."
When asked if he were in the best shape of his career, Karlsson said: "I think my early years I was in much better shape when I was 21, 22, and I could do lot more off the ice without worrying about other things. Right now, I feel good.
"I think the time I have put in this summer has been in the right way. I've done what I needed to do to get my body ready to play another 82 games and then some, and do it at the highest possible level that I can.