Lekhwiya had a 2-1 advantage from the first-leg in this all-Qatar Round 16 clash played at the Abdullah Bin Khalifa Stadium in Doha.
The home side opened the scoring through Youssef Msakni in the 13th minute when Sebastian Soria crossed from the right for the ball to go past goalkeeper Saad Al Sheeb and then falling for Msakni to tap-in.
But in the 34th minute Lekhwiya were down to 10-men when defender Ahmed Yasser was sent off for a second bookable offence for a foul on Khalfan Ibrahim.
Brazilian star Muriqui scored for Sadd from the free-kick to make the score 1-1 and change the match completely.
Sadd dominated after getting the equaliser with Lekhwiya trying to defend their one-goal advantage.
In the 63rd minute Sadd took the lead when midfielder Ali Asadalla scored with a shot into the far corner after a decent pass from the left by Nadir Belhadj.
Coach Michael Laudrup brought on Ismaeel Mohamad to replace Vladimir Weiss in the 64th minute.
That move proved to be the master stroke as the striker scored the goal that got Lekhwiya through to the quarter-finals.
The goal was scored in the 83rd minute when Karim Boudiaf made a run down the left crossed to Mohamad, who controlled it to score with a left-footed shot.