Singh was speaking at a function to celebrate conferment of Japan's Imperial Decoration on him.
Recalling his association with Japan since 1962, the former revenue secretary said nobody at that time thought the country would emerge as an important investment source for India.
"I have seen a transformation in Indo-Japanese relations of an unbelievable order...India is now high on radar of Japanese investment.
"I have seen in my lifetime this transformative relationship from being a merely trading partner to becoming important strategic partners. As we go along this will be an important shaping feature of the 21st century," he added.
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The government of Japan announced the foreign recipients of the 2016 Spring Imperial Decorations on April 29, 2016.
The Order of the 'Rising Sun, Gold and Silver Star' was conferred on Singh, in recognition of his contribution to strengthening relations and promoting mutual understanding between Japan and India, said Kenji Hiramatsu, Ambassador of Japan.
Singh is a former Rajya Sabha MP.