Ms Singh was removed by the government with seven months to go before retirement, and replaced by S Jaishankar, the former envoy to the US, just after a successful visit by President Barack Obama.
"It was a decision that had already been taken and nothing I could have done would have made a difference," she said.
She revealed that she was summoned on Wednesday by External Affairs Minister Sushma Swaraj, who told her that Prime Minister Narendra Modi wanted Mr Jaishankar as his Foreign Secretary.
"I said, as instructed by the prime minister, I hereby request for early retirement," said Ms Singh.
"Why has it become so low and dirty?" the 60-year-old officer questioned, and told NDTV that she wanted to set the record straight.
"I felt I needed to this because I want an honourable exit. I don't mean a tenure as Ambassador or a tenure in UPSC. I mean an exit which is in keeping with what I have been as an individual, what I have been as an officer, as a Foreign Secretary," she said.
Ms Singh said she deserved more credit than she had been given for driving foreign policy over the past eight months, and referred to what was described as a major takeaway of President Obama's visit this week - the agreement on the nuclear deal.
"Shall I start by claiming credit for working on the nitty gritty of the recent understandings we arrived at on the civil nuclear deal with the US ? The liability and the administrative issues? Believe me, I was fully involved in guiding the discussions, taking decisions on the line to take, on what to do and what not to do. I coordinated very closely with the Prime Minister's Office," she said.
"This government has charged a great deal on the foreign policy front over the past 8 months. None of this would have happened with the PM (Prime Minister Narendra Modi) or the EAM (External Affairs Minister Sushma Swaraj) operating on their own, in a vacuum."
The officer confirmed reports that she had been offered an exit last year. "I was sounded out in December as to whether I'd be interested in a three or five year Constitutional position. I declined because of my belief that my responsibility was to the Ministry and to my Service," she told NDTV.