Kewell bemoans missed chances with Champions League hopes on knife edge
The Japanese team host Ulsan Hyundai in next Wednesday's second leg with the South Koreans ahead thanks to Lee Dong-gyeong's 19th-minute goal.
Yokohama are in the last four of Asia's top club competition for the first time.
Kewell, the former Liverpool and Leeds star, saw his side create several chances at Ulsan Munsu Football Stadium.
But they failed to score despite mustering 18 shots, while the hosts were a constant menace on the break.
"I thought we did extremely well, I felt we dominated the match," said Kewell, who replaced fellow Australian Kevin Muscat as Yokohama coach at the start of the year.
"Yes, they had chances, but I felt those chances came from our own kind of mistakes," the Asian Football Confederation website quoted the former international forward as saying.
Kewell felt that South Korean international goalkeeper Jo Hyeon-woo kept Ulsan in it.
"We are disappointed that we didn't score as we had enough chances," said Kewell.
"These chances were not lucky, they were well worked. The Ulsan keeper is fantastic, he pulled off a few saves.
"But we have to be more clinical."
The winner of Yokohama and Ulsan will face either Saudi Arabia's Al Hilal or Al Ain of the United Arab Emirates in next month's two-legged final.
Al Ain won 4-2 at home in their semi-final first leg with Morocco striker Soufiane Rahimi hitting a hat-trick.