Leaders Porto were held to a 0-0 draw at third-placed Sporting in a lively Portuguese 'clasico' on Saturday in which both teams wasted several clear-cut chances.
The champions now have 34 points from 14 matches. Sporting are on 28 points.
Porto are undefeated in the league for 53 games, their last reverse coming in February 2010 when they lost 3-0 at Sporting.
Argentine centre-back Nicolas Otamendi headed narrowly wide for Porto in the 24th minute but Sporting also threatened five minutes later when Brazilian defender Anderson Polga forced compatriot Helton to make a flying save.
Both sides then went close late on.
Porto's Uruguay international Alvaro Pereira cleared substitute Marat Izmailov's effort off the line after 83 minutes and in stoppage-time Otamendi unintentionally blocked team-mate James Rodriguez's goal-bound shot.comments