Derlis Gonzalez???s 11th-minute strike appeared to have settled the tie in favour of the Swiss champions but Danilo???s penalty 10 minutes from time, after Walter Samuel was adjudged to have handled in the area, ensured it would end level.
Group H winners Porto, entered the tie as favourites after an impressive group-stage campaign but it was Basel who got the first goal as Gonzalez ran from deep to collect Fabian Frei???s through ball before sliding it under the on-rushing Fabiano. The goalscorer picked up a head injury in the process of scoring and was substituted shortly after.
Porto had strong claims for a penalty denied midway through the first half when Jackson Martinez appeared to be wrestled to the floor, but referee Mark Clattenburg was unmoved by the claims.
The Portuguese champions were further aggrieved just minutes into the second half as Clattenburg disallowed Casemiro???s strike following consultation with additional assistant Kevin Friend. The Premier League officials ruled both Marcano and Maicon interfered with Basel goalkeeper Tomas Vaclik from an offside position.
Porto got the decision they were looking for when Clattenburg penalised Samuel and pointed to the spot in the 79th-minute. Right-back Danilo stepped up and scored what could be a vital away goal to level the tie on the night.
Porto were the brighter of the two sides in the opening exchanges but failed to convert their dominance as Basel opened the scoring against the run of play.
Gonzalez ghosted in behind Alex Sandro to meet Frei???s pass and, after taking the ball in his stride, he raced in between the two Porto centre backs before sliding the ball under the on-rushing Fabiano.
Fabiano and Gonzalez collided as both competed for the ball and the latter was replaced by Davide Calla having gingerly returned to play after the initial impact.
Porto continued in the fashion with which they had started the game and appeared unfazed after falling behind, as Danilo looked for a quick response with a pop-shot from 20 yards but Vaclik was on hand to deny the full-back.
Martinez had strong claims for a penalty waved away after he and Marcano tussled for the ball in the area, but Basel, courtesy of the referee???s decision entered the break in front.
Porto flew out of the blocks in the second half and appeared to have levelled the tie as Casemiro volleyed home from close range after Maicon???s header was parried by Vaclik into the goal mouth.
With the celebrations almost over, Clattenburg blew his whistle having consulted his fellow official and the goal was ruled out for offside. Replays showed two Porto players obstructing Vaclik???s attempts to save the effort.
Porto were the brighter of the two teams and they were handed a lifeline late on as Samuel was penalised for blocking a low cross into the box.
Danilo drilled the ball into the bottom corner of the goal as Porto grabbed the initiative in the tie ahead of the second leg at the second leg at the Estadio do Dragao on March 10.