Sporting Lisbon 1 Porto 1: Visitors draw again
Jonathan Silva put the hosts at the Estadio Jose Alvalade in Lisbon ahead early, only for Mouhamadou-Naby Sarr to score a second-half own goal.
It was a good result for league leaders Benfica, who sit a point clear of Porto ahead of Saturday's trip to Estoril.
Porto and fifth-placed Sporting remain unbeaten, but draws have proven costly as Benfica threaten to edge away.
Sporting made the dream start as Silva gave them a second-minute lead.
Porto goalkeeper Fabiano came out to collect what appeared to be an overhit pass, but he was instead left stranded as Silva got on the end of a cross to head into an open goal.
The visitors were able to earn a point in fortuitous circumstances as French defender Sarr looked to intercept a cross in the 56th minute, only to put it past his own goalkeeper Rui Patricio.