Seoul took control of their AFC Champions League quarter-final tie with a 3-1 first-leg win over Shandong Luneng on Wednesday.
Goals from Dejan Damjanovic, Park Chu-young and Adriano gave the Korean side the victory, though Walter Montillo netted an important away goal.
Damjanovic set up Park for a glorious early chance that the former Arsenal striker scuffed wide, but a role-reversal gave Seoul the lead after 19 minutes, as Damjanovic headed in Park's pinpoint cross.
Park made amends for his miss just after the half-hour mark, drilling home following Cho Chan-ho's long pass, but Shandong seized a lifeline in the tie as Montillo cleverly steered a free-kick into the bottom corner.
Seoul restored their two-goal cushion in the second half, as Damjanovic's delightful backheel split the defence and found Adriano, who finished with aplomb.
Jin Jingdao was sent off for a second yellow card with nine minutes remaining to compound a miserable first leg for Felix Magath's side, who have it all to do in the return fixture next month.
Al Nasr have one foot firmly in the semi-finals after they dispatched El Jaish 3-0 at Abdullah bin Khalifa Stadium.
Wanderley struck just before half-time for the visitors, prodding into the unguarded net after Jonathan Pitroipa had beaten goalkeeper Khalifa N'Diaye to the ball.
Pitroipa doubled his side's advantage on 54 minutes, arrowing a low shot beyond the reach of N'Diaye and into the far corner from the edge of the area.
El Jaish failed to take advantage of a number of chances to halve the deficit, with Romarinho twice thwarted by Ahmed Shambih, and Wanderley fired in Salem Saleh's pass with five minutes to play to give Al Nasr a formidable lead to take back to Dubai.