Porto's advantage at the Primeira Liga summit evaporated as they were held to a 0-0 draw by defending champions Benfica in controversial circumstances at Estadio do Dragao on Friday.
League leaders Porto were denied two first-half penalties and saw a legitimate goal by Hector Herrera ruled out for an incorrect offside decision against Vincent Aboubakar - referee Jorge Sousa not using VAR on any of those occasions.
Sergio Conceicao's side were unable to breakthrough despite Andrija Zivkovic being sent off after two yellow cards within six minutes of coming on as a substitute, with Sporting CP's 1-0 victory over Belenenses earlier in the day earning them a share of first.
Alejandro Grimaldo appeared to foul Moussa Marega in the area inside eight minutes but escaped punishment, while Luisao was similarly fortunate after appearing to handle a cross from the Mali international on the stroke of half-time.
There was further drama before the hour-mark when Aboubakar fired Sergio Oliveira's knockdown at Benfica goalkeeper Bruno Varela and Herrera turned home the rebound, but the goal was incorrectly chalked off.
Zivkovic replaced Franco Cervi in the 76th minute and two yellow cards in the space of 116 seconds resulted in his dismissal, but Porto goalkeeper Jose Sa had to be alert to keep out Filip Krovinovic in the 85th minute.
Marega failed to turn a left-wing cross home from the edge of the six-yard box and, after sparking a confrontation between the two sets of players as he objected to time-wasting from the Benfica dugout, headed over a glorious stoppage-time chance that meant the spoils were shared.