Four days after winning the BMW PGA Championship, Manassero's lowest score of the year tied him with Pablo Larrazabal of Spain on the Bro Hof Slott course.
"I've never started that well in my career, that just seemed a little strange," the Italian said.
"It was a guess how I would wake up this morning as I had been pretty tired up until last night but I felt good and I could see that from the first tee when I hit good shots."
With his promising young career to date, Manassero is hoping to follow in the footsteps of the late Seve Ballesteros.
He wore green pants in honor of the Spanish great for Sunday's victory, his fourth on the European Tour at the age of 20 years and 37 days. Ballesteros was 20 years and three months old when he captured a fourth European title in winning the 1977 Swiss Open.
"Seve Ballesteros has always been my idol given his personality and what he did for golf and the tournaments that he won, and the way he won them was different to other golfers," Manassero said.
If Manassero, who also wears navy blue during the week of an event in further honor of Ballesteros, should succeed this Sunday in Sweden he would be more than 13 months younger than when the Spanish legend won a fifth Tour title.
Two-time Tour winner Larrazabal capped his round with four birdies over his opening nine holes.
"I played great to be five under par after 10 holes and then six under par after 14 and even after a bogey at 16 I managed to birdie the last," said the Barcelona-born Larrazabal.
"So I played steady and I played safe, but then I played aggressive where I could, so seven birdies in a round is good for me."
Donaldson, the reigning Irish Open and Abu Dhabi HSBC Championship title holder, collected his five birdies over his outward nine and then had nine pars in succession in his 67.