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.
Nacional 01 Elisson 03 Felipe Lopes 04 Danielson 05 Nuno Pinto 07 Mihelic (Diego - 46' ) 08 Skolnik (Elizeu - 46' ) 22 Joao Aurelio Booked 88 Luis Alberto 09 Mateus 18 Rondon 20 Costa Booked (Candeias - 69' ) Substitutes 12 Valverde 33 Luis Neto 06 Todorovic 11 Candeias 66 Elizeu 10 Diego 23 Oliver Birmingham City 01 Myhill 02 Carr Booked 03 Murphy 04 Caldwell 06 Ridgewell Booked 23 Spector 24 Davies 07 Burke 11 Beausejour Booked 22 Redmond 39 Wood Substitutes 13 Doyle 37 Hubbins 18 Fahey 20 Valles Prat 17 Rooney 25 Asante 36 Jervis
Ref: Milorad Mazic