A free kick from Willian in first-half injury time has ensured that Chelsea go into the break on level terms with Porto.
It took just two minutes for Porto's Yacine Brahimi to force an early save out of Asmir Begovic in the Chelsea goal, but up the other end, Cesc Fabregas saw Iker Casillas react sharply to push his strike past the post.
Shortly before the quarter-hour mark, the Blues should have edged in front after Diego Costa and Willian combined to set up Pedro, but the Spaniard was unable to test Casillas despite finding himself in a promising position.
Clear-cut opportunities were few and far between as the game progressed past 30 minutes, but Vincent Aboubakar did manage to get a shot away as he curled the ball just over the crossbar.
However, with six minutes of the half remaining, Andre gave the home side the lead when he was well placed to score after Begovic had initially saved from Brahimi.
Porto looked to have secured the advantage at the break but deep into added-on time, Willian sent a free kick into the corner to change the tone of Jose Mourinho's team talk.