The 34-year-old, who was at AC Milan at the time, was set to follow Carlo Ancelotti to Stamford Bridge when he was appointed Blues boss that summer.
However, Pirlo has said that he was persuaded to stay at the San Siro by Milan owner Silvio Berlusconi.
"It was August 2009 and I'd reached agreement with Chelsea, the club where Ancelotti had just come in as manager," Pirlo said in his autobiography.
"Carlo was like a father and a teacher for me, a kind, friendly man who knew how to make things fun. I'd spent the best years of my career with him. If you're a player who wants to get on and give everything, you won't find anyone better than him.
"Ancelotti and I spoke a fair bit on the phone. He wanted to bring me to London at all costs, and cost was indeed the last hurdle still to be overcome. Insurmountable, as it transpired."
Pirlo joined current club Juventus in 2011.