The South Korean was at 10-under 203 after a round of five birdies and three bogeys at the Woo Jeong Hills Country Club. American Anthony Kim (67), Englishman Ian Poulter (69) and South Koreans Bae Sang-moon (67) and Kim Dae-sub (69) were tied for second.
Before playing this week, my goal was to finish in the top-10, Kim Wi-joong said. But now I feel confident enough for a win. Ill be playing with some famous golfers tomorrow so Ill try to focus on my game on the final charge on Sunday.
The top five are four shots clear of the rest of the field and are set to fight out the title between them.
Anthony Kim was unhappy with his swing and putting, but put himself in strong contention by bouncing back after a 2-over 73 in the second round.
My game is not where I want it to be but I fought hard and Im looking forward to Sunday, he said. I am going to play smart until the last four or five holes tomorrow and then I will see where I am in the tournament and go from there.
Poulter, who has some terse words for Anthony Kim after being bumped by the celebrating American at the Ryder Cup, got two bogeys in the opening three holes but gradually improved through the round.
I hope to play golf, have fun and try to win this tournament, Poulter said. Its a good course and there are some tough holes out there.
Kim Dae-sub is aiming for his third Korea Open title after winning it as an amateur in 1998 and 2001.
I had no pressure today and I feel really good, he said. I am just a little disappointed when I missed birdie putts on the last hole and some other holes on the front nine.