His second duet with Jang Hyejin, was released on July 14, 2011 throughout Korea's digital charts. "I Am Behind You" is from the album "Cooperation Part 1" . The song has a fantastic harmony and is mixed with the lyrics of secretly loving someone and wondering if that person could possibly be feeling the same way about them. On July 18, 2011, "For One Day" was released. It is from the OST SBS historical drama "Warrior Baek Dong Soo" . The song explores an emotional farewell from a lover. Yesung performed this live for the first time on October 6, 2011 at the K.R.Y Concert in Nanjing.