Klitschko, who holds the IBF, IBO, WBO and WBA crown had used the 28-year-old Pianeta as a sparring partner last year.
The 36-year-old Ukrainian's record stands at 59 wins and three defeats after his three successful title defences in 2012, including a November victory over Poland's Mariusz Wach.
Pianeta, who at 1.93 metres is five centimetres shorter than his opponent, is still undefeated with 28 wins and one draw.
"I have a lot of respect for Pianeta because he is young and has a big fighting spirit," the 36-year-old Klitschko said.
Klitschko's older brother, Vitali, holds the WBC crown.
"I was briefly his sparring partner last year and I noted where his weaknesses are and how best to exploit them," said Pianeta.
"I understand many see me as an outsider and that is fine but I will knock Klitschko out and turn the world of boxing on its head."
Later that evening in the United States, Floyd Mayweather Jr takes on Robert Guerrero in Las Vegas, the feature attraction of this year's Cinco de Mayo weekend of boxing.