Ricardo Carvalho and Fabio Coentrao scored to secure a third straight qualifying win for Portugal, after Nemanja Matic had equalised with a spectacular bicycle kick at Estadio da Luz.
The victory takes them above Denmark and back to top spot in the group, while Serbia remain bottom.
Albania came from a goal down to see off Armenia 2-1 in Group I's other fixture.
Gibraltar were able to celebrate their first competitive goal in a heavy Group D loss at Hampden Park.
In David Wilson's first outing as Gibraltar boss, the minnows fell behind to Shaun Maloney's penalty for Scotland before Lee Casciaro registered their historic goal.
However, a second Maloney penalty, a Steven Naismith strike and Steven Fletcher's hat-trick saw Gordon Strachan's men run out comfortable winners.
Like Scotland, Germany piled the pressure on Group D leaders Poland as they beat Georgia 2-0 in Tbilisi.
Goals from Marco Reus and Thomas Muller earned the world champions' first victory of 2015 and there was a boost for Scotland and Germany later in the day when Poland were held to a 1-1 draw by the Republic of Ireland.
Having led for much of the game thanks to Slawomir Peszko's 26th-minute strike, Poland were denied by Shane Long's injury-time strike in Dublin.
Romania were made to work hard for a 1-0 win over the Faroe Islands that kept them top of Group F, Claudiu Keseru grabbing the only goal, while Kyle Lafferty's brace ensured Northern Ireland remain hot on Romania's heels.
The Norwich City loanee scored a quickfire first-half brace for Northern Ireland in a 2-1 win over Finland.
Hungary's clash with Greece in Budapest ended goalless.
