An early goal by Rafinha gave Ulsan Hyundai a slender advantage in their AFC Champions League quarter-final against Al Hilal. The Brazilian striker found the net after nine minutes at the Ulsan Munsu Football Stadium, with a shot which took a wicked deflection off Hilal captain Majed Al Marshadi on its way past goalkeeper Abdullah Mohammed.
The Saudi visitors had their chances to level the score and Ulsan were grateful to goalkeeper Kim Young-Kwang, who made a fine save right at the end to deny Abdulaziz Al Dosari.
The two sides will meet again in the return match at the Prince Faisal Bin Fahd Stadium in Riyadh on 3 October.
Both sides had been unbeaten on their run to the last eight and it was Ulsan, the only South Korean side left in this year's competition, who started the stronger.
They crafted an opening after eight minutes when Rafinha found Lee Keun-Ho alone on left of the Al Ahli box, but the South Korean international's attempt to curl the ball into the net was denied by Abdullah Mohammed's diving save.
But just a minute later, the Korean side did take the lead. Adil Hermach's loose pass found its way to an unmarked Lee, who slipped the ball to Rafinha whose shot went in with the aid of a big deflection off Al Marshadi.
Al Hilal's Korean striker Yoo Byung-Soo went close to equalising on the half-hour with a near-post header that was brilliantly kept out by 'keeper Kim.
Just before the break, Abdulla Al Zori laid the ball off to an unmarked Yassir Al Shahrani but the Al Hilal defender fired wastefully over with the goal at his mercy.
Early in the second half, Mohammad Al Nakhli saw his long-range effort palmed away by Kim and from the resulting corner, Yoo headed wide.
Former Saudi Arabia captain Yasser Al Qahtani came on as a late replacement for Yoo but Al Hilal still could not equalise as Kim did well in stoppage time to foil Al Dosari, before Salman Al Faraz put the rebound wide.