Birmingham earned a valuable draw in Portugal in their first major European game for 49 years.
And the Blues could have even taken an advantage into the Europa League play-off second leg game after hitting the woodwork three times.
Steven Caldwell headed against the bar from a Jean Beausejour corner in the first half.
Striker Chris Wood hit the post twice, the first from a free-kick and then a header from Chris Burke's delivery.
The Portuguese side made a good start when Mateus da Costa was denied by a well-timed challenge by defender Curtis Davies in the area.
The Championship side fought back with Davies bringing a good save out of home goalkeeper Elisson, before Caldwell and Wood both struck the woodwork.
Wood struck the post with a header again and then fired an effort across the face of goal.
The Blues came close to snatching victory in the last five minutes when Caldwell headed Beausejour's free-kick a foot over the bar.