Tough Champions League start for Wanderers
Santalab struck after just 43 seconds to give the Wanderers a dream start before they were taught a harsh-lesson by the 2012 ACL champions.
So resolute and stingy throughout their A-League life, the home side were guilty of three poor defensive errors which handed the initiative to Ulsan.
Giant striker Kim Shin-Wook, Ko Chang-Hyun and Kang Minsoo were the beneficiaries of the errors to get their name on the scoresheet and settle the issue in front of more than 11,000 fans.
And the manner of the defeat is bound to deflate much of the hype and excitement that had been building for Tony Popovic's side heading into their maiden ACL campaign.
Given Ulsan's domestic season has yet to get underway, the Wanderers were always going to try to jump the visitors at the start but no one could have predicted just how quick they would pounce.
Santalab came up with a superb finish but Shinji Ono deserves much of the credit for the lead-up work.
The Japanese international produced an audacious flick-pass into the path of Santalab, who hit a sweet controlled volley into the bottom corner.
Ono was involved again 16 minutes later as the home side went close to doubling their lead, his free-kick met perfectly by Nikolai Topor-Stanley but the defender put his header wide.
The heavens opened soon after and with it came a change in the momentum of the contest.
Shin-Wook had been proving a real handful for the Wanderers defence and he came up with the equaliser 10 minutes before the break.
Jerome Polenz and Topor-Stanley both made a meal of trying to clear the ball, allowing Shin-Wook to clinically side-foot past a stranded Ante Covic.
The home side was then made to pay for more lax defending just before half-time, again failing to deal with a high ball as Chang-Hyun pounced with a superb curling half-volley.
Just like in the first half it was the Wanderers who started the second on the front foot, with Mark Bridge's 20-yard shot sailing just over the bar.
The attacker's next effort on the hour almost won the home side a penalty, with the ball appearing to strike the arm of Minsoo.
But Omani referee Abdullah Mohamed waved away appeals for a spot kick and just six minutes later Minsoo made an even more decisive play at the opposite end.
The defender was on hand to poke home from close range after another poor headed clearance, this time from Iacopo La Rocca.
The Wanderers never stopped looking for a way back into the game but couldn't break down the well-organised Ulsan rearguard and they will need to regroup before their trip to China against Guzhou Renhe in a fortnight.
Western Sydney Wanderers 1 (Santalab 1)
Ulsan Hyundai 3 (Shin-Wook 35, Chang-Hyun 43, Minsoo 67)
Crowd: 11,212 @ Parramatta Stadium