Borussia Dortmund have taken a 1-0 lead at halftime in Porto courtesy of an own goal by Iker Casillas.
With the marker, Dortmund have opened up a 3-0 lead on aggregate, and the crucial road goal means that Porto will have to score four times in order to advance to the next round.
While Porto are unleashing an all-out attack, Dortmund are using possession and patience as their strategy. The Portuguese side have looked to send long balls into the box in an attempt to create space and opportunities, but Dortmund's defense has remained stalwart.
BVB have held 63% of the possession over the first 45 minutes. The have also won 66% of the duels, including a 53% success rate on aerial battles.
Porto meanwhile hold a hefty advantage in corners, leading 7-1, and have won 100% of their tackles.
Both teams have only taken one shot on target, with both being stopped.
BVB scored in 23rd minute when a Marco Reus volley was stopped by Casillas, and Pierre-Emerick Aubameyang sent the rebound off the bar and onto the legs of Casillas, sending the ball spinning into the net.
Porto looked to have equalized off a corner, but the header was stopped by the lunging Roman Burki.
Evandro had another good chance for Porto midway through the half as his streaking run ended with a shot rolling just wide of the net.
The distribution in the game has been better overall from Dortmund as they are completing passes at 82.5% accuracy to Porto's 74.1%.
Referee Mark Clattenburg went to his pocket just one time, showing a yellow card to Porto's Evandro in the 14th minute for a tackle on Julian Weigl.