Ulsan will finish second in Group F behind Shanghai SIPG, who drew 1-1 at home to Kawasaki Frontale.
And on a great night for South Korean football, the 2016 winners Jeonbuk also clinched their berth with a 2-0 Group E win at Kashiwa Reysol in Japan.
China’s Tianjin Quanjian left it late to join them in the last 16 from Group E with the only goal against Kitchee in Hong Kong coming in the 89th minute from 30 million euro French import Anthony Modeste.
Ulsan were gifted a comical opening goal before the floodgates opened against the A-League side.
Just 12 minutes had elapsed when striker Junior Negrao threw out a hopeful boot to block a clearance from Melbourne goalkeeper Lawrence Thomas, who was mortified to see the ball ricochet back over his head from the Brazilian and into the empty net.
Lim Jong-eun got the second with a cool left-foot volleyed finish and Mislav Orsic thumped home a third before half-time to put the home side in complete control.
Kim Seung-jun made it 4-0 for Ulsan 10 minutes after the break and Junior bagged his second before a brief fightback from Melbourne.
The visitors scored twice in two minutes through Kenny Athiu and Kosta Barbarouses. But any fleeting hopes of an against-the-odds fightback were snuffed out when the Croatian Orsic thumped in his second to make it 6-2.
Jeonbuk were grateful to another Brazilian striker, Ricardo Lopes, who got them up and running in front of a vociferous Kashiwa home support with a scrambled finish in the 16th minute.
Hugely popular veteran Korean striker Lee Dong-gook sealed qualification with a cool finish in the 77th minute.
The 38-year-old had been on the pitch for just 10 minutes when he sweetly struck a left-foot shot past Kazushige Kirihata to leave Kashiwa’s qualification hopes at that stage dangling by a thread.
Kashiwa needed Kitchee to defeat the big-spending Tianjin in the later match but the Chinese Super League side broke Japanese hearts in the 89th minute when Modeste nodded in from close range to knock out both Kitchee and Kashiwa.
Tianjin are two points behind the Korean side and will play Kashiwa next week while Jeonbuk face Kitchee with those results determining who will finish top of the group.
In Shanghai, burly striker Hulk blazed a first-half penalty over the bar and fellow Brazilian Oscar was largely anonymous as the already-qualified SIPG laboured to a 1-1 against Group F stragglers Kawasaki Frontale.
SIPG eventually opened the scoring through Brazilian Elkeson’s 68th-minute strike but it was cancelled out six minutes later by a Kei Chinen close-range header.
The draw was still enough to assure SIPG of finishing top of the group with Ulsan guaranteed second place with one round of matches remaining next week.
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