Antoine Griezmann found the net but Atletico Madrid failed to put pressure on leaders Barcelona as they were held 1-1 at home by Girona on Saturday.
The France forward has been heavily linked with a move to Barcelona, who on Saturday released a statement denying they had reached an agreement to sign the 26-year-old this summer.
He put that speculation to one side to fire Atletico ahead at the Wanda Metropolitano after being set up by Diego Costa's headed pass in the 34th minute.
Girona played the second half with backup goalkeeper Gorka Iraizoz after starter Yassine Bounou asked to be substituted with an injury late in the first half.
But Atletico failed to press for a second goal, and a defensive slip helped Girona level in the 73rd after the hosts failed to clear a free kick despite multiple attempts.
Portu got free in the area to equalise from close range after Bernardo Espinosa headed forward a poor clearance by substitute Koke.
It was Portu's eighth goal of the season for Girona, top-flight newcomers who have thus far exceeded expectations and risen to eighth place.
Atletico, who also conceded two late goals at home to lose 2-1 at Sevilla in the Copa del Rey earlier this week, sit eight points behind Barca, who have played a game fewer.
Information from The Associated Press and Press Assocation was used in this report.