"We could have scored one or two more goals but this advantage gives us confidence for the second leg.
"Nothing is settled yet. It's 50/50. I cannot promise that we will qualify but I will do all I can to achieve it.''
Sweden coach Erik Hamren pledged a more attacking approach from his team next Tuesday.
"We have work to do but we will do all we can to win,'' he said.
The first-half was predictably nervy with opportunities at a premium.
Portugal's best chance came after just six minutes when Raul Meireles picked out Joao Moutinho who was clear one-on-one with Sweden goalkeeper Andreas Isaksson.
But Isaksson forced Mountinho out wide and the Portuguese player could only find the side netting.
A minute later, it was the visitors' turn to threaten.
Mikael Lustig sprinted down the right flank before unleashing a fine cross for Johan Elmander but the Norwich City striker could only prod the ball just past the right post of Rui Patricio in goal.
Ibrahimovic was subdued with his only contribution being a decoy at a free-kick from which Kim Kallstrom missed the target.
Portugal looked the more dangerous early in the second half with Helder Postiga and Pepe both unable to force the ball past Isaksson in a frantic scramble.
The ball broke out to Ronaldo whose drive flew over the top.
But the Real Madrid star was to have the final word on the night.
-AAP