Two goals from Denilson and a strike for substitute Kim Jae-sung in the second half looked to have completed the unlikely turnaround for Pohang following last week’s 3-1 defeat in Tashkent.
Bunyodkor substitute Victor Karpenko fired home with two minutes remaining to pull his side back into the tie before Ristic struck the winner 11 minutes into the first half of extra-time.
Bunyodkor manager Luiz Felipe Scolari appeared to set his side out to protect their first-leg advantage but despite spending the majority of the first half controlling possession in the visitors’ territory, Pohang failed to find the target.
No Byung-jun should have handed the home side the lead eight minutes before the break but after Denilson ran the length of the pitch beating four defenders in the process and seeing his shot saved, the striker fired over with the goal at his mercy.
Pohang grabbed a lifeline just 30 seconds into the second half as the home side finally broke the deadlock as Ristic took advantage after Bunyodkor defender Sakhob Jurayev failed to clear the ball out of the area to set up Kim for the first- half substitute to fire home.
Bunyodkor goalkeeper Ignatiy Nesterov denied Pohang three times in quick succession five minutes before the hour mark, but from the resulting corner Denilson headed home to level the tie on aggregate.
The goal seemed to spur Bunyodkor into life and the visitors began to press forward as Rivaldo saw his well-struck effort from the edge of the area saved by Pohang goalkeeper Shin Hwa-yong with 17 minutes remaining.
Pohang, though, looked to have secured their place in the last four minutes later as a swift counterattack resulted in Ristic finding Denilson and the Brazilian calmly curled past an exposed Nesterov.
The celebrations, though, were halted with two minutes remaining as Karpenko’s speculative 25-yard effort skipped past Shin to force extra-time.
But after a tense start to the first half of extra-time, Ristic headed over Nesterov to send the home fans into raptures.