Manassero, who claimed his fourth European Tour title at the BMW PGA Championship last weekend, continued his good form at Bro Hof Slott Golf Club, north of Stockholm.
He was joined at the top of the leaderboard by Pablo Larrazabal, who was also at six under after the first round.
A shot back were Sweden's 2011 champion Alexander Noren and Fredrik Andersson Hed, along with Chris Lloyd, Simon Wakefield and 2009 runner-up Jamie Donaldson.
Manassero had six straight birdies to get his tournament off to a flying start.
"I've never started that well. It was a great start. I was making putts and to start like that was strange because I'm not used to it," Manassero said.
"I've been tired, but I woke up and I felt good. It's a tough course when the wind picks up, so I think every afternoon is going to be hard."
Manassero is bidding to become the first player to win consecutive European Tour titles since Branden Grace at the beginning of the 2012 season.
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- Fredrik Andersson Hed