After a breakthrough season in 2008, during which he won twice, claimed $4.6 million and was one of the stars of the American Ryder Cup team, Kim went without a victory in 2009.
He began his comeback with a 3-1 record last month at the Presidents Cup and he opened the match play event with a solid showing.
"I haven't had this much fun playing golf in quite a long time," said Kim, whose victory over Casey ended the top-seeded Englishman's chances of winning this week.
By losing twice on the first day, Casey was denied a chance to move into the semifinals even with one match remaining Friday. Sergio Garcia and Martin Kaymer were also eliminated on the first day.
They were victims of a new format that separated the 16-man field into four groups. After each player competes in a match against the other three in his group, the four group winners advance to the semifinals.
All matches are played over a full 18 holes so the total number of holes won can be used in case of ties.
Oliver Wilson also won two matches to take the lead in his group, defeating Garcia and Kaymer. Kim will reach the semifinals unless he suffers at least a four-hole loss to Scott Strange Friday.