Paulo Fonseca's team looked to be on their way to a fifth consecutive win in the league until Luis Leal struck with 10 minutes left to earn Estoril a share of the spoils.
Lica had opened the scoring for Porto in the 26th minute, but their lead did not last long as the hosts were awarded a penalty nine minutes later.
Evandro stepped up and calmly slotted his spot-kick past Helton.
Jackson Martinez's strike just after the hour mark then looked to have won the game for Porto, but Leal's late strike meant it ended level.
Benfica benefited from Porto's draw as they extended their unbeaten run to five games in all competitions to close the gap at the top by beating Vitoria Guimaraes 1-0.
Jorge Jesus' side had looked strong throughout the match and they were given a lift when David Addy was sent off for a second bookable offence just after the hour.
Benfica took advantage, and after applying pressure on the Vitoria goal, they finally broke the deadlock in the 73rd minute through Oscar Cardozo.
In the day's other game, Nacional sent Academica to their third defeat in the opening five games with a 1-0 victory at Estadio da Madeira.
Mario Rondon's 32nd-minute strike was enough to secure Nacional's first home victory of the campaign.
