In a statement posted on his official website, the 23-year-old Brazilian said it is "with disappointment" that he has been served notice that he will be replaced.
"But, at the same time, I feel a sense of relief for the end of the worst period of my career," said Piquet, who slammed his team boss and personal manager Flavio Briatore.
He said he received "no attention at all" during his debut season in 2008, and claims that occasionally he would be threatened with the sack "15 minutes before qualifying and races".
Piquet revealed that he was promised equal conditions with Fernando Alonso this season but remained Renault's clear number two, with a performance-based contract "requiring me to score 40 per cent of Fernando Alonso's points" by mid-season.
But Piquet claims that in four of the first nine races in 2009, his R29 was a specification behind Alonso's car, and clearly blames Briatore.
"I always believed that having a manager was being a part of a team and having a partner. A manager is supposed to encourage you, support you, and provide you with opportunities. In my case it was the opposite.
"Flavio Briatore was my executioner," the Brazilian said.