Maxi Pereira returned to haunt his former club by scoring the equaliser as Porto drew 1-1 with title rivals Benfica in a dramatic Classico at Estadio da Luz.
After the hosts had gone ahead through Jonas' early penalty, Pereira - who made more than 200 appearances for Benfica between 2007 and 2015 - struck an equaliser soon after half-time.
The result means Benfica stay top of the Primeira Liga table by one point and remain on course to secure a fourth consecutive title for first time in their history, but Porto's fighting spirit - evident in abundance on the night - will likely ensure the Eagles are pushed all the way to the end of the season.
The game exploded into life when Porto defender Felipe brought down Jonas in the penalty area after five minutes and the Benfica striker - celebrating his 33rd birthday - hit the spot-kick low into the middle of the net to put the home side ahead.
Tempers flared moments later when Jonas shoulder-barged Porto coach Nuno Espirito Santo on the touchline, renewing bad blood that dates from the time when the pair worked together at Valencia and Nuno sold the Brazilian.
Benfica's Kostas Mitroglou tested Iker Casillas with a powerful shot on the turn after 28 minutes but the former Real Madrid goalkeeper was equal to it, and Porto drew level after four minutes of the second half.
After Benfica failed to clear their lines Pereira volleyed the ball into the net to make it 1-1 and Casillas ensured it stayed that way with a brilliant one-handed save from a Jonas shot five minutes later to leave the two great rivals tightly matched with seven games remaining.